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كتاب السهو
22  
Forgetfulness in Prayer
(1)
Chapter: What is said regarding the Sahw: If someone gets up (for the third rak'a without sitting) after the second rak'a of a compulsory salat (prayer)
(1)
باب مَا جَاءَ فِي السَّهْوِ إِذَا قَامَ مِنْ رَكْعَتَىِ الْفَرِيضَةِ
Sahih al-Bukhari 1224

Narrated `Abdullah bin Buhaina:

Allah's Messenger (ﷺ) once led us in a prayer and offered two rak`at and got up (for the third rak`a) without sitting (after the second rak`a). The people also got up with him, and when he was about to finish his prayer, we waited for him to finish the prayer with Taslim but he said Takbir before Taslim and performed two prostrations while sitting and then finished the prayer with Taslim.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكُ بْنُ أَنَسٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَبْدِ الرَّحْمَنِ الأَعْرَجِ، عَنْ عَبْدِ اللَّهِ ابْنِ بُحَيْنَةَ ـ رضى الله عنه ـ أَنَّهُ قَالَ صَلَّى لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم رَكْعَتَيْنِ مِنْ بَعْضِ الصَّلَوَاتِ ثُمَّ قَامَ فَلَمْ يَجْلِسْ، فَقَامَ النَّاسُ مَعَهُ، فَلَمَّا قَضَى صَلاَتَهُ وَنَظَرْنَا تَسْلِيمَهُ كَبَّرَ قَبْلَ التَّسْلِيمِ فَسَجَدَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ ثُمَّ سَلَّمَ‏.‏
Reference : Sahih al-Bukhari 1224
In-book reference : Book 22, Hadith 1
USC-MSA web (English) reference : Vol. 2, Book 22, Hadith 315
  (deprecated numbering scheme)