Aulus Gellius, Attic Nights 1.15.1

“Those light-weight, annoying and pointless talkers who, though they cannot rely on any strong foundation, pour out lolling, liquid words, are correctly believed to draw only as deep as the lips and not the heart. Indeed, most people say that the tongue should not be free but should be guided by lines tied to the deepest part of the chest and the heart, as if by a ship’s captain. But still you may see certain men who toss around words without any semblance of judgment, but instead with a certainty so great and profound that even while they are speaking they do not seem to understand that they speak.

Homer has his Ulysses, however,–a man suffused with wise eloquence–move his voice not from his mouth but from his chest. This depiction is not so much about the sound and style of his voice as it is indicative of the considerable weight of the thoughts conceived within. And Homer also said quite appropriately that teeth are a wall built to contain immature and dangerous words—not just so that the watchful guardian of the heart could restrain them, but that they may be stopped by a guardhouse of sorts positioned at the mouth. The Homeric lines which I mentioned above are: “But when he released the great voice from his chest” (Il.3.221) and “What kind of word has escaped the bulwark of your teeth”? (Il. 4.350)

1 Qui sunt leves et futtiles et inportuni locutores quique nullo rerum pondere innixi verbis uvidis et lapsantibus diffluunt, eorum orationem bene existimatum est in ore nasci, non in pectore; linguam autem debere aiunt non esse liberam nec vagam, sed vinclis de pectore imo ac de corde aptis moveri et quasi gubernari. 2 Sed enim videas quosdam scatere verbis sine ullo iudicii negotio cum securitate multa et profunda, ut loquentes plerumque videantur loqui sese nescire.

3 Ulixen contra Homerus, virum sapienti facundia praeditum, vocem mittere ait non ex ore, sed ex pectore, quod scilicet non ad sonum magis habitumque vocis quam ad sententiarum penitus conceptarum altitudinem pertineret, petulantiaeque verborum coercendae vallum esse oppositum dentium luculente dixit, ut loquendi temeritas non cordis tantum custodia atque vigilia cohibeatur, sed et quibusdam quasi excubiis in ore positis saepiatur. 4 Homerica, de quibus supra dixi, haec sunt:
ἀλλ’ ὅτε δὴ ὄπα τε μεγάλην ἐκ στήθεος εἵη (Il.3.221)

et:
… ποῖόν σε ἔπος φύγεν ἕρκος ὀδόντων; (4.350)

This topic is covered in other authors like Plutarch. It seems obvious to say but still merits saying that in our modern world where everyone has a platform and is encouraged to speak, the choice not to speak becomes ever so rarer and precious.