Musnad Ahmad 10:2

Ibrahim ibn Sa‘d narrated to us, Muhammad ibn Ishaq narrated to me, from Makhul, from Kurayb, from Ibn ‘Abbas, that ‘Umar said to him: "O young man, have you heard from the Messenger of Allah (may Allah bless him and grant him peace), or from any of his companions, what a man should do if he is in doubt regarding his prayer?" He said: While he was like that, ‘Abd al-Rahman ibn ‘Awf approached and said: "What are you two discussing?" ‘Umar said: "I asked this young man: Has he heard from the Messenger of Allah (may Allah bless him and grant him peace), or from any of his companions, what a man should do if he is in doubt regarding his prayer?" ‘Abd al-Rahman said: "I heard the Messenger of Allah (may Allah bless him and grant him peace) say: 'If one of you is in doubt regarding his prayer and does not know whether he has prayed one [rak'ah] or two, let him count it as one. And if he does not know whether he has prayed two or three, let him count it as two. And if he does not know whether he has prayed three or four, let him count it as three. Then, when he has finished his prayer, let him prostrate two prostrations while he is sitting, before he gives the salutation.'"