What a person should do if he stands up after two rak'ahs and forgets to say the tashahh
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 قَبْلَ التَّسْلِيمِ ثُمَّ سَلَّمَ .
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 waiting for him to say the taslim, he said the takbir and prostrated twice while sitting, before the taslim. Then he said the taslim."