Musannaf Ibn Abi Shaybah 4479 — A man who prays and does not know

Abd al-Karim said: If Sa'id bin al-Musayyib and Abu Ubaydah became uncertain about how much of the prayer they had prayed, and could not tell whether they had prayed three rak'ahs or four, both of them would make the prostrations of forgetfulness before giving the salam.