Riyad as-Salihin, Book 18, Hadith 1656 (In-Book) ⓘ

Masruq (May Allah be pleased with him) said: We visited 'Abdullah bin Mas'ud (May Allah be pleased with him) and he said to us: O people! He who has the knowledge of any matter may convey it to the others. And he who has no knowledge, thereof, should say: "Allahu a'lam (Allah knows better)." It is a part and parcel of knowledge that a man who has no knowledge of a matter should say: "Allah knows better." Allah said to His Prophet:"Say (O Muhammad): 'No wage do I ask of you for this (the Qur'an), nor am I one of the Mutakallifun (those who pretend and fabricate things which do not exist)."' (38:86)[Al-Bukhari].

وعن مسروق قال ‏:‏ دخلنا على عبدالله بن مسعود رضي الله عنه فقال ‏:‏ يا أيها الناس من علم شيئا فليقل به، ومن لم يعلم، فليقل ‏:‏الله أعلم، فإن من العلم أن يقول الرجل لما لا يعلم‏:‏ الله أعلم ‏.‏ قال الله تعالى لنبيه صلى الله عليه وسلم ‏:‏‏{‏قل ما أسألكم عليه من أجر وما أنا من المتكلفين‏}‏‏.‏ ‏(‏‏(‏رواه البخاري‏)‏‏)‏‏.‏