What a person should do if he stands up after two rak'ahs and forgets to say the tashahh

Sunan an-Nasa'i · Book 13 · Hadith 1222

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَبْدِ الرَّحْمَنِ الأَعْرَجِ، عَنْ عَبْدِ اللَّهِ ابْنِ بُحَيْنَةَ، قَالَ صَلَّى لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم رَكْعَتَيْنِ ثُمَّ قَامَ فَلَمْ يَجْلِسْ فَقَامَ النَّاسُ مَعَهُ فَلَمَّا قَضَى صَلاَتَهُ وَنَظَرْنَا تَسْلِيمَهُ كَبَّرَ فَسَجَدَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَ…

It was narrated that Abdullah bin Buhainah said: "The Messenger of Allah (ﷺ) led us in praying two rak'ahs, then he stood up and did not sit, and the people stood up with him. When he finished the prayer, and we were wai…

Sunan an-Nasa'i · Book 13 · Hadith 1223

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ هُرْمُزَ، عَنْ عَبْدِ اللَّهِ ابْنِ بُحَيْنَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ قَامَ فِي الصَّلاَةِ وَعَلَيْهِ جُلُوسٌ فَسَجَدَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ التَّسْلِيمِ ‏.‏

It was narrated from 'Abdullah bin Buhainah that: The Messenger of Allah (ﷺ) stood up during the prayer when he should have sat, so he prostrated twice while sitting, before the taslim.