Both at-Tirmidhiyy (in his book Al-Jami^) and the hafidh, Ibn Muhammad Ibn Abu Bakr al-Qurashiyy (in his book, As-Samt, which means "The Silence") narrated from Prophet Muhammad that he said:

These words of the Prophet mean: <<Say something good or else keep silent.>> This advice is important to follow because giving judgments in the Religion without knowledge is among the ways which lead a person to be a loser in the Hereafter. Indeed, this matter is among the top reasons leading people to enter Hellfire. The one who says something is "halal" (permissible) or "haram" (forbidden) in the Religion without relying on a valid religious permission to do so is sinful and deserves the severe torture in Hellfire