Musnad Ahmad 30:685

Ibrahim narrated to us, Rabah narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, the devil comes to one of you in his prayer, so he does not know whether he added or subtracted. So if one of you finds that, let him perform two prostrations."