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 ﷺ:

"Say (O Muhammad ﷺ): '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 ﷺ :{قل ما أسألكم عليه من أجر وما أنا من المتكلفين}. ((رواه البخاري)).