Hadith Q1029
وَالْمُضِيِّ فِي الصَّلَاةِ إِذَا اسْتَوَى الْمُصَلِّي قَائِمًا، وَإِيجَابِ سَجْدَتَيِ السَّهْوِ عَلَى فَاعِلِهِ
And when the person praying stands up straight, he should continue the prayer; two prostrations of forgetfulness are obligatory upon such a person.
Hadith 1029
Sayyiduna Ibn Buhayna (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in prayer—we think it was the ‘Asr prayer—then when he was in the second rak‘ah, he stood up and did not sit (for tashahhud, but stood up). Then, before saying the salam (while sitting for tashahhud), the Messenger of Allah (peace and blessings be upon him) performed two prostrations of forgetfulness while sitting.
Hadith 1030
Sayyiduna Abdullah bin Buhaina (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in one of the prayers, and in the second rak‘ah he stood up (without sitting for tashahhud). He was reminded by saying "Subhan Allah," but he continued with his prayer until he finished it and only the salam was left. So, before the salam, he performed two prostrations while sitting.