Musnad Ahmad 31:698

Yunus ibn Muhammad narrated to us, Fulayh narrated to us, from Zayd ibn Aslam, from ‘Ata’ ibn Yasar, from Abu Sa‘id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you is in doubt regarding his prayer, and does not know how much he has prayed, let him build upon certainty. Then, when he is certain that he has completed it, let him prostrate twice before he gives the salutation. For if his prayer was odd, it becomes even, and if it was even, that is a humiliation for the devil."