Musnad Ahmad 30:704

'Abd al-Razzaq narrated to us, Ma'mar and 'Abd al-A'la informed us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The devil comes to one of you and confuses him in his prayer, so he does not know whether he added or subtracted. If one of you finds that, let him perform two prostrations while he is sitting."