Musannaf Ibn Abi Shayba 3:2043

Abu Bakr narrated to us, he said: Muhammad ibn Mus’ab narrated to us, from al-Awza’i, from Yahya ibn Abi Kathir, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If he does not know whether he added or decreased, then let him prostrate two prostrations while he is sitting."