Sunan an-Nasa'i, Volume 2, Book 13, Hadith 1256 (DarusSalam) ⓘ

It was narrated from 'Abdullah that: The Prophet led them in praying Zuhr with five (rak'ahs). They said: 'You prayed five.' So he prostrated twice after he had said the taslim, while he was sitting.

أَخْبَرَنَا عَبْدَةُ بْنُ عَبْدِ الرَّحِيمِ، قَالَ أَنْبَأَنَا ابْنُ شُمَيْلٍ، قَالَ أَنْبَأَنَا شُعْبَةُ، عَنِ الْحَكَمِ، وَمُغِيرَةَ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ صَلَّى بِهِمُ الظُّهْرَ خَمْسًا فَقَالُوا إِنَّكَ صَلَّيْتَ خَمْسًا ‏.‏ فَسَجَدَ سَجْدَتَيْنِ بَعْدَ مَا سَلَّمَ وَهُوَ جَالِسٌ ‏.‏