Musnad Ahmad 30:168

Sufyan narrated to us, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, attributing it to the Prophet (peace and blessings of Allah be upon him): "The devil comes to one of you while he is in his prayer and confuses him, until he does not know how much he has prayed. So, whoever finds anything of that, let him perform two prostrations while he is sitting."