Musnad Ahmad 31:829

'Ali ibn 'Ayyash narrated to us, Muhammad ibn Mutarrif narrated to us, Zayd ibn Aslam narrated to us, from 'Ata' ibn Yasar, from Abu Sa'id al-Khudri, who said: The Prophet, peace and blessings of Allah be upon him, said: "If one of you is in doubt regarding his prayer, let him cast away the doubt and build upon certainty, and let him perform two prostrations. For if it was five, he will have made them even with them, and if he had prayed four, they will be a humiliation for the Shaytan."