al-Imâm ash-Shâfi'î said: "When one desires to talk, then it is upon him to think before he speaks. If there is beneficial good in what he will say, then he should speak. If he has doubt about that, then he must not speak until he clears that doubt."
Qass bin Sâ'adah and Aktham bin Sayfî once met and one of them said to the other: "How many defects were you able to find in the son of Adam?" The other responded: "They are too numerous to count. However, the defects that I was able to account for numbered eight thousand. I also discovered one thing which, if put into practice, would hide all of these defects." He asked: "What is it?" He responded: "Guarding the tongue."
al-Fudayl bin 'Iyâd said: "Whoever limits his speech to be in accordance with his actions will minimize his speech on that which doesn't concern him."
al-Imâm ash-Shâfi'î said to one of his students: "Do not speak about things that do not concern you, for indeed, every time that you speak a word, it takes control of you and you do not have control of it!"
'Abdullâh bin Mas'ûd said: "There is nothing that deserves to be imprisoned more than the tongue."
Others have stated: "The tongue is like a savage beast; if you do not lock it up, it will set out against you."
Abul-Qâsim al-Qushayrî said: "Remaining silent is salvation and that is a fundamental principle. Staying quiet at the proper time is a characteristic of manhood, just as speaking when it is proper to do so is from the most honorable of manners."
He also said: "Whoever remains silent concerning the truth, then he is a silent devil."
Guard your tongue, O mankind
And do not let it bite you, for indeed it is a snake.
How many people in the graveyards are there who were killed by their tongues?
Whoever fears meeting Allah is truly the brave one...
[Translated from 'Kitâb al-Adhkâr' of al-Imâm an-Nawawî]