Musannaf Ibn Abi Shaybah 4483 — A man who prays and does not know

Abd al-Aziz bin Suhayb said: Sayyiduna Anas bin Malik (may Allah be pleased with him) sat in the third rak'ah, so the people behind him said "Subhan Allah". He stood up and completed the fourth rak'ah. When he gave the salam, he made the two prostrations of forgetfulness. Then he turned to the people and said, "When you become uncertain, do like this."