Hadith: O people, he who knows about something, let him say it. And he who does not know should say: Allah knows best. It is part and parcel of knowledge that a man who does not know something should say: Allah knows best

عن مسروق، قال: دخَلْنَا على عبد الله بن مسعود رضي الله عنه فقال: يا أيها الناس، من عَلِم شيئا فَلْيَقُلْ به، ومن لم يَعْلَم، فَلْيَقُلْ: الله أعلم، فإن من العلم أن يقول لما لا يَعْلَم: الله أعلم. قال الله تعالى لنبيه صلى الله عليه وسلم : (قل ما أسألكم عليه من أجر وما أنا من المتكلفين).
[صحيح] - [رواه البخاري]

Masrūq reported: We visited ‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) and he said to us: 'O people, he who knows about something, let him say it. And he who does not know should say: 'Allah knows best.' It is part and parcel of knowledge that a man who does not know something should say: 'Allah knows best.' Allah said to His Prophet: {Say: 'I do not ask you for it any payment, and I am not of the pretentious.'} [Sūrat Sād: 86]"
[Authentic hadith] - [Narrated by Bukhari]

Explanation

If a person is asked about something and he knows the answer, he should share his knowledge with the people and not conceal it. But if he is asked about something that he doesn't know then he should say: "Allah knows best", and not to make up an answer to pretend that he knows. It is a sign of having knowledge to say: "Allah knows best" in areas that one does not have knowledge in. The person who does so is the one with true knowledge, because he does not overestimate himself and his status, and knows that he has no knowledge concerning that matter, so he refers knowledge of it to Allah by saying: "Allah knows best". Another reporion in Sahīh Muslim reads: "It is more knowledgeable for one of you to say: ‘Allah a’lam’ about things you do not know", meaning: it is better for his knowledge and more beneficial for him to say regarding what he does not know: "Allah knows best". Then Ibn Mas’ood used the following verse from the Qur'an to support his statement: {Say: "I do not ask you for the Qur'an any payment, and I am not of the pretentious"} [Sād: 86], meaning: I do not ask you for reward or wages in return for the revelation that I bring to you; rather, my sole intention is to guide you to goodness and to invite you to worship Allah. {and I am not of the pretentious}, meaning: I am not one of those who make matters difficult for you or those speaking without knowledge. In brief, it is not permissible for a person to issue a Fatwa unless he is permitted to do so. If Allah wills for him to be an Imām for the people, issue religious decisions for them, and guide them to the straight path, indeed, he will be so. And if Allah does not will that for him, his boldness and daring in issuing a Fatwa will not be of benefit to him; rather, it will be a test and trial for him in this world and the next life.