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)

Narrated 'Abdullah bin Buhaina :

Allah's Apostle once led us in a prayer and offered two Rakat and got up (for the third Raka) without sitting (after the second Raka). 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 ﷺ 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.