Imam Shafi' said: "When one wishes to say something, 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 that doubt is removed."
al-Fudayl ibn 'Iyaad said: "Whoever limits his speech to be in accordance with his actions will (surely) lessen his speech on that which doesn't concern him."
Imam Shafi' adviced 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 any control over it!"
Abdullah ibn Mas'ood said: "There is nothing that deserves to be imprisoned more than the tongue."
Abul-Qaasim al-Qushayree said: "Remaining silent is salvation and that is a fundamental principle. Staying quiet at the appropriate time is a characteristic of manhood, just as speaking when it is right to do so is from the most honorable of manners."