"O you who believe! Avoid much suspicion, for some suspicions are a sin. Do not spy on one another, nor backbite one another. Would one of you love to eat the flesh of his dead brother? Nay, you would abhor it, [so similarly, avoid backbiting]. And fear Allah. Indeed, Allah is Most Forgiving, Most Merciful." Qur'an, [49:12]
"And do not follow that of which you do not have knowledge. Indeed, the hearing, the sight and the heart - [you] will be asked about all of those." Qur'an, [17:36]
"He does not utter a [single] word, except that there is, with him, [an angel] ready and waiting [to record it]." Qur'an, [50:18]
Imam Nawawi says "It is obligatory for every sane adult to guard his tongue against talking, except when it contains a clear benefit. If talking and remaining silent are of equal benefit, it is sunnah to abstain, [emphasis added] for permissible talking might lead to something undesirable or forbidden, as in fact is very often the case, and nothing matches safety."
On the authority of Abu Hurayrah :
"Whoever believes in Allah and the Last Day should say [something] good, or he should keep silent." [Bukhari, Muslim, Ahmad, Tirmidhi, Ibn Majah]
... Mukhtasar Minhaj al-Qasideen, Ibn Qudamah