"The Prophet (S) stood for the Zuhr prayer, and he had a sitting to perform, so when he completed his Salat, he performed two prostrations, saying the Takbir for each prostration. So he was sitting before saying the Salam, and the people prostrated with him in place of the sitting he forgot."
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ شِهَابٍ، عَنِ الأَعْرَجِ، عَنْ عَبْدِ اللَّهِ ابْنِ بُحَيْنَةَ الأَسَدِيِّ، حَلِيفِ بَنِي عَبْدِ الْمُطَّلِبِ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَامَ فِي صَلاَةِ الظُّهْرِ وَعَلَيْهِ جُلُوسٌ فَلَمَّا أَتَمَّ صَلاَتَهُ سَجَدَ سَجْدَتَيْنِ يُكَبِّرُ فِي كُلِّ سَجْدَةٍ وَهُوَ جَالِسٌ قَبْلَ أَنْ يُسَلِّمَ وَسَجَدَهُمَا النَّاسُ مَعَهُ مَكَانَ مَا نَسِيَ مِنَ الْجُلُوسِ . قَالَ وَفِي الْبَابِ عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ .
Tirmizi
The Book on Salat (Prayer) • Hadith 391