Musannaf Ibn Abi Shaybah 4555 — Those who used to say: two

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 said the salam, he performed the two prostrations of forgetfulness. Then he turned towards the people and said: "When you are in doubt (or make a mistake), do like this."