Musnad Ahmad 31:393

'Arim narrated to us, Sa'id ibn Zayd narrated to us, he said: 'Ali ibn al-Hakam narrated to us, he said: Abu Nadra narrated to us, from Abu Sa'id, and he attributed it to the Prophet, may Allah bless him and grant him peace, that he said: "If a man becomes confused in his prayer, and does not know whether he added or subtracted, then let him prostrate two prostrations while he is sitting."