Musnad Ahmad 30:3638

'Abd al-Malik ibn 'Amr narrated to us, Hisham narrated to us, from Yahya, from Abu Salamah, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "When the call to prayer is made, the Shaytan turns his back, passing wind so that he does not hear the Adhan. When the Adhan is finished, he returns. When the Iqamah is called, he turns his back. When the Iqamah is finished, he returns to whisper between a man and his heart—or he said: his soul—saying: 'Remember such and such, remember such and such,' regarding things he was not remembering, until the man remains not knowing how much he has prayed. So if any of you does not know how much he has prayed, three or four, let him perform two prostrations while he is sitting."