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
The pious, knowledgeable people said it is a very good act for the person to say: "I do not know," when it is applicable to do so. If one does not know the religious judgment on a particular case, he should not venture his own opinion on it. Once, when Imam ^Aliyy was asked about something which he did not know, he said, "I do not know." Then he added, "How soothing it is for me to say ‘I do not know’ when I do not know." Imam ^Aliyy meant that he felt comfortable not to answer a religious question when he did not know the answer to it. We should take heed and follow the words of Imam ^Aliyy in this matter. He was one of the most honored Companions of the Prophet--known for his high level of knowledge and comprehension in the Religion.
Once, when Prophet Muhammad, sallallahu ^alayhi wa sallam, was asked what the best and the worst places were, he answered:
His words mean: "I do not know. I will ask Jibril." When the Prophet asked Jibril, Jibril also said, "I do not know." After a period of time, Jibril returned to Prophet Muhammad with the answer which Allah revealed to him: "The worst places are the markets and the best places are the mosques." This is a sahih hadith related by Imam Muslim and others.
The Prophet, sallallahu ^alayhi wa sallam, taught us a very important lesson by his response. It is not shameful for one to reply, "I do not know" when it is applicable, but it is shameful to speak about religious matters without knowledge. Only Allah knows everything. For the Muslim to be safe on the Day of Judgment, he should speak only about what he knows. If one needs an answer to a Religious question, he should ask one more knowledgeable than himself or a trustworthy Muslim who can ask another pious, knowledgeable Muslim. This is the way the knowledge of the Religion is taught, and this is the way the knowledge is preserved.
