Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) said: 'If anyone of you is uncertain about his prayer, let him put aside uncertainty and act upon that which is certain. When he has made sure his prayer is complete, then let him prostrate twice. Then if his prayer was complete, that (extra) Rakah will be counted as voluntary, and if his prayer was lacking, that Rakah will complete his prayer, and the two prostrations will rub the Satan's nose in the dust.'" Sahih