Musnad Ahmad 30:576

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, Ibn Shihab informed me, from Abu Salamah ibn 'Abd al-Rahman, from Abu Hurayrah, that he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The devil comes to one of you while he is in his prayer, and he confuses him, until he does not know how much he has prayed. So if he finds that, let him prostrate two prostrations while he is sitting."