Sahih al-Bukhari, Volume 4, Book 54, Hadith 505 (USC-MSA) ⓘ

Narrated Abu Huraira: The Prophet said, "When the call for the prayer is pronounced, Satan takes to his heels, passing wind with noise, When the call for the prayer is finished, he comes back. And when the Iqama is pronounced, he again takes to his heels, and after its completion, he returns again to interfere between the (praying) person and his heart, saying to him. 'Remember this or that thing.' till the person forgets whether he has offered three or four rak`at: so if one forgets whether he has prayed three or four rak`at, he should perform two prostrations of Sahu (i.e. forgetfulness).

حَدَّثَنَا مُحَمَّدُ بْنُ يُوسُفَ، حَدَّثَنَا الأَوْزَاعِيُّ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ إِذَا نُودِيَ بِالصَّلاَةِ أَدْبَرَ الشَّيْطَانُ وَلَهُ ضُرَاطٌ، فَإِذَا قُضِيَ أَقْبَلَ، فَإِذَا ثُوِّبَ بِهَا أَدْبَرَ، فَإِذَا قُضِيَ أَقْبَلَ، حَتَّى يَخْطِرَ بَيْنَ الإِنْسَانِ وَقَلْبِهِ، فَيَقُولُ اذْكُرْ كَذَا وَكَذَا‏.‏ حَتَّى لاَ يَدْرِي أَثَلاَثًا صَلَّى أَمْ أَرْبَعًا فَإِذَا لَمْ يَدْرِ ثَلاَثًا صَلَّى أَوْ أَرْبَعًا سَجَدَ سَجْدَتَىِ السَّهْوِ ‏" ‏‏.‏