"And what is right speech? Abstaining from lying, from divisive speech, from abusive speech, & from idle chatter: This is called right speech."
— SN 45.8
"Monks, a statement endowed with five factors is well-spoken, not ill-spoken. It is blameless & unfaulted by knowledgeable people. Which five?
"It is spoken at the right time. It is spoken in truth. It is spoken affectionately. It is spoken beneficially. It is spoken with a mind of good-will."
— AN 5.198
"For the person who transgresses in one thing, I tell you, there is no evil deed that is not to be done. Which one thing? This: telling a deliberate lie."
— Iti 25
"One should speak only that word by which one would not torment oneself nor harm others. That word is indeed well spoken.
"One should speak only pleasant words, words which are acceptable (to others). What one speaks without bringing evils to others is pleasant."
— Thag 21
"And how is one made pure in four ways by verbal action?
"There is the case where a certain person, abandoning false speech, abstains from false speech. When he has been called to a town meeting, a group meeting, a gathering of his relatives, his guild, or of the royalty, if he is asked as a witness, 'Come & tell, good man, what you know': If he doesn't know, he says, 'I don't know.' If he does know, he says, 'I know.' If he hasn't seen, he says, 'I haven't seen.' If he has seen, he says, 'I have seen.' Thus he doesn't consciously tell a lie for his own sake, for the sake of another, or for the sake of any reward. Abandoning false speech, he abstains from false speech. He speaks the truth, holds to the truth, is firm, reliable, no deceiver of the world.
"Abandoning divisive speech he abstains from divisive speech. What he has heard here he does not tell there to break those people apart from these people here. What he has heard there he does not tell here to break these people apart from those people there. Thus reconciling those who have broken apart or cementing those who are united, he loves concord, delights in concord, enjoys concord, speaks things that create concord.
"Abandoning abusive speech, he abstains from abusive speech. He speaks words that are soothing to the ear, that are affectionate, that go to the heart, that are polite, appealing & pleasing to people at large.
"Abandoning idle chatter, he abstains from idle chatter. He speaks in season, speaks what is factual, what is in accordance with the goal, the Dhamma, & the Vinaya. He speaks words worth treasuring, seasonable, reasonable, circumscribed, connected with the goal.
"This is how one is made pure in four ways by verbal action."
"And how is right view the forerunner? One discerns wrong speech as wrong speech, and right speech as right speech. And what is wrong speech? Lying, divisive tale-bearing, abusive speech, & idle chatter. This is wrong speech...
"One tries to abandon wrong speech & to enter into right speech: This is one's right effort. One is mindful to abandon wrong speech & to enter & remain in right speech: This is one's right mindfulness. Thus these three qualities — right view, right effort, & right mindfulness — run & circle around right speech."
— MN 117
 "In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others, he does not say them.
 "In the case of words that the Tathagata knows to be factual, true, unbeneficial, unendearing & disagreeable to others, he does not say them.
 "In the case of words that the Tathagata knows to be factual, true, beneficial, but unendearing & disagreeable to others, he has a sense of the proper time for saying them.
 "In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, but endearing & agreeable to others, he does not say them.
 "In the case of words that the Tathagata knows to be factual, true, unbeneficial, but endearing & agreeable to others, he does not say them.
 "In the case of words that the Tathagata knows to be factual, true, beneficial, and endearing & agreeable to others, he has a sense of the proper time for saying them. Why is that? Because the Tathagata has sympathy for living beings."
— MN 58
— Sn 3.3
[The Buddha speaks to his son, Rahula:] "Whenever you want to perform a verbal act, you should reflect on it: 'This verbal act I want to perform — would it lead to self-affliction, to the affliction of others, or to both? Is it an unskillful verbal act, with painful consequences, painful results?' If, on reflection, you know that it would lead to self-affliction, to the affliction of others, or to both; it would be an unskillful verbal act with painful consequences, painful results, then any verbal act of that sort is absolutely unfit for you to do. But if on reflection you know that it would not cause affliction... it would be a skillful verbal action with happy consequences, happy results, then any verbal act of that sort is fit for you to do.
"While you are performing a verbal act, you should reflect on it: 'This verbal act I am doing — is it leading to self-affliction, to the affliction of others, or to both? Is it an unskillful verbal act, with painful consequences, painful results?' If, on reflection, you know that it is leading to self-affliction, to the affliction of others, or to both... you should give it up. But if on reflection you know that it is not... you may continue with it.
"Having performed a verbal act, you should reflect on it... If, on reflection, you know that it led to self-affliction, to the affliction of others, or to both; it was an unskillful verbal act with painful consequences, painful results, then you should confess it, reveal it, lay it open to the Teacher or to a knowledgeable companion in the holy life. Having confessed it... you should exercise restraint in the future. But if on reflection you know that it did not lead to affliction... it was a skillful verbal action with happy consequences, happy results, then you should stay mentally refreshed and joyful, training day and night in skillful mental qualities."
— MN 61
"Whereas some priests and contemplatives, living off food given in faith, are addicted to talking about lowly topics such as these — talking about kings, robbers, ministers of state; armies, alarms, and battles; food and drink; clothing, furniture, garlands, and scents; relatives; vehicles; villages, towns, cities, the countryside; women and heroes; the gossip of the street and the well; tales of the dead; tales of diversity [philosophical discussions of the past and future], the creation of the world and of the sea, and talk of whether things exist or not — he abstains from talking about lowly topics such as these. This, too, is part of his virtue.
"Whereas some priests and contemplatives, living off food given in faith, are addicted to debates such as these — 'You understand this doctrine and discipline? I'm the one who understands this doctrine and discipline. How could you understand this doctrine and discipline? You're practicing wrongly. I'm practicing rightly. I'm being consistent. You're not. What should be said first you said last. What should be said last you said first. What you took so long to think out has been refuted. Your doctrine has been overthrown. You're defeated. Go and try to salvage your doctrine; extricate yourself if you can!' — he abstains from debates such as these. This, too, is part of his virtue."
— DN 2
"Monks, do not wage wordy warfare, saying: 'You don't understand this Dhamma and discipline, I understand this Dhamma and discipline'; 'How could you understand it? You have fallen into wrong practices: I have the right practice'; 'You have said afterwards what you should have said first, and you have said first what you should have said afterwards'; 'What I say is consistent, what you say isn't'; 'What you have thought out for so long is entirely reversed'; 'Your statement is refuted'; 'You are talking rubbish!'; 'You are in the wrong'; 'Get out of that if you can!'
"Why should you not do this? Such talk, monks, is not related to the goal, it is not fundamental to the holy life, does not conduce to disenchantment, dispassion, cessation, tranquillity, higher knowledge, enlightenment or to Nibbana. When you have discussions, monks, you should discuss Suffering, the Arising of Suffering, its Cessation, and the Path that leads to its Cessation. Why is that? Because such talk is related to the goal... it conduces to disenchantment... to Nibbana. This is the task you must accomplish."
— SN 56.9
"There are these ten topics of [proper] conversation. Which ten? Talk on modesty, on contentment, on seclusion, on non-entanglement, on arousing persistence, on virtue, on concentration, on discernment, on release, and on the knowledge & vision of release. These are the ten topics of conversation. If you were to engage repeatedly in these ten topics of conversation, you would outshine even the sun & moon, so mighty, so powerful — to say nothing of the wanderers of other sects."
— AN 10.69
"O bhikkhus, a bhikkhu who desires to admonish another should do so after investigating five conditions in himself and after establishing five other conditions in himself. What are the five conditions which he should investigate in himself?
 "Am I one who practices purity in bodily action, flawless and untainted...?
 "Am I one who practices purity in speech, flawless and untainted...?
 "Is the heart of goodwill, free from malice, established in me towards fellow-farers in the holy life...?
 "Am I or am I not one who has heard much, who bears in mind what he has heard, who stores up what he has heard? Those teachings which are good alike in their beginning, middle, and ending, proclaiming perfectly the spirit and the letter of the utterly purified holy life — have such teachings been much heard by me, borne in mind, practiced in speech, pondered in the heart and rightly penetrated by insight...?
 "Are the Patimokkhas [rules of conduct for monks and nuns] in full thoroughly learned by heart, well-analyzed with thorough knowledge of their meanings, clearly divided sutta by sutta and known in minute detail by me...?
"These five conditions must be investigated in himself.
"And what other five conditions must be established in himself?
 "Do I speak at the right time, or not?
 "Do I speak of facts, or not?
 "Do I speak gently or harshly?
 "Do I speak profitable words or not?
 "Do I speak with a kindly heart, or inwardly malicious?
"O bhikkhus, these five conditions are to be investigated in himself and the latter five established in himself by a bhikkhu who desires to admonish another."
— AN V (From The Patimokkha, Ñanamoli Thera, trans.)