Masruq reported,

We entered upon Abdullah bin Masud, and he said, "People, whoever knows something, let him speak of it, and whoever does not know, let him say, 'Allah knows best.' For it is part of knowledge that one says about what he does not know, 'Allah knows best.' Allah عز وجل said to His Prophet, 'Say, I ask of you no reward for it, nor am I of the pretenders' [Sad 38:86]."

Azami 13186

Sahih

عن مسروق قال

دخلنا على عبد الله بن مسعود قال: يا أيها الناس! من علم شيئا فليقل به، ومن لم يعلم فليقل: الله أعلم، فإن من العلم أن يقول لما لا يعلم: الله أعلم. قال الله عز وجل لنبيه: {قل ما أسألكم عليه من أجر وما أنا من المتكلفين}

الأعظمي 13186صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Tafsir of the great Quran
Chapter
Tafsir of Surah Sad
Section
His saying, "But if they turn away, then upon him is only that with which he has been charged, and upon you is that with which you have been charged. And if you obey him, you will be guided. And there is not upon the Messenger"
Numbering
Jami al-Kamil 13186 (2nd edition)
Azami 13186