Language:
كتاب السهو
22
Forgetfulness in Prayer
(6)
Chapter: When a person forgets whether he has offered three or four Rak'a
(6)
باب إِذَا لَمْ يَدْرِ كَمْ صَلَّى ثَلاَثًا أَوْ أَرْبَعًا سَجَدَ سَجْدَتَيْنِ وَهْوَ جَالِسٌ

Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "When the call for prayer is made, Satan takes to his heels passing wind so that he may not hear the Adhan and when the call is finished he comes back, and when the Iqama is pronounced, Satan again takes to his heels, and when the Iqama is finished he comes back again and tries to interfere with the person and his thoughts and say, "Remember this and that (which he has not thought of before the prayer)", till the praying person forgets how much he has prayed. If anyone of you does not remember whether he has offered three or four rak`at then he should perform two prostrations of Sahu while sitting.

حَدَّثَنَا مُعَاذُ بْنُ فَضَالَةَ، حَدَّثَنَا هِشَامُ بْنُ أَبِي عَبْدِ اللَّهِ الدَّسْتَوَائِيُّ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا نُودِيَ بِالصَّلاَةِ أَدْبَرَ الشَّيْطَانُ وَلَهُ ضُرَاطٌ حَتَّى لاَ يَسْمَعَ الأَذَانَ، فَإِذَا قُضِيَ الأَذَانُ أَقْبَلَ، فَإِذَا ثُوِّبَ بِهَا أَدْبَرَ فَإِذَا قُضِيَ التَّثْوِيبُ أَقْبَلَ حَتَّى يَخْطِرَ بَيْنَ الْمَرْءِ وَنَفْسِهِ يَقُولُ اذْكُرْ كَذَا وَكَذَا مَا لَمْ يَكُنْ يَذْكُرُ حَتَّى يَظَلَّ الرَّجُلُ إِنْ يَدْرِي كَمْ صَلَّى، فَإِذَا لَمْ يَدْرِ أَحَدُكُمْ كَمْ صَلَّى ثَلاَثًا أَوْ أَرْبَعًا فَلْيَسْجُدْ سَجْدَتَيْنِ وَهْوَ جَالِسٌ ‏"‏‏.‏
Reference : Sahih al-Bukhari 1231
In-book reference : Book 22, Hadith 9
USC-MSA web (English) reference : Vol. 2, Book 22, Hadith 323
  (deprecated numbering scheme)