ذِكْرُ وَصْفِ هَذِهِ الصَّلَاةِ الَّتِي سَجَدَ فِيهَا ﷺ سَجْدَتَيِ السَّهْوِ لِلْحَالِ الَّتِي وَصَفْنَاهَا قَبْلَ السَّلَامِ
The Messenger of Allah ﷺ stood up in the noon prayer (though) he hadith sit (after the two rak'ahs). When he completed the prayer he performed two prostrations and said," Allah is the Most Great" in each prostration, while he was sitting before pronouncing salutation, and the people performed prostration along with him. That was a compensation for he had forgotten to observe jalsa (after two rak'ahs). (Using translation from Muslim 570b)
«أَنَّ رَسُولَ اللَّهِ ﷺ قَامَ فِي صَلَاةِ الظُّهْرِ وَعَلَيْهِ جُلُوسٌ فَلَمَّا أَتَمَّ صَلَاتَهُ سَجَدَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ أَنْ يُسَلِّمَ وَسَجَدَهُمَا النَّاسُ مَعَهُ مَكَانَ مَا نَسِيَ مِنَ الْجُلُوسِ»