Musnad Ahmad 3:52

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Musarrah ibn Ma'bad narrated to us, from Yazid ibn Abi Kabshah, from Uthman ibn Affan, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, I prayed and I did not know whether I had performed an even number of rak'ahs or an odd number." The Messenger of Allah, may Allah bless him and grant him peace, said: "Beware that the devil plays with you in your prayer. Whoever among you prays and does not know whether he performed an even number or an odd number, let him prostrate two prostrations, for they are the completion of his prayer."