(13) The Book of Forgetfulness (In Prayer)
كتاب السهو
(26) Chapter: What a person should do if he prays five (rak'ahs)
(26) باب مَا يَفْعَلُ مَنْ صَلَّى خَمْسًا
Sunan an-Nasa'i 1254 
It was narrated that 'Abdullah said:
"The Prophet(ﷺ) prayed Zuhr with five rak'ahs, and it was said to him: 'Has something been added to the prayer?' He said: 'Why are you asking?' They said: 'You prayed five.' So he turned around and prostrated twice."
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَمُحَمَّدُ بْنُ بَشَّارٍ، - وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا يَحْيَى، عَنْ شُعْبَةَ، عَنِ الْحَكَمِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ صَلَّى النَّبِيُّ صلى الله عليه وسلم الظُّهْرَ خَمْسًا فَقِيلَ لَهُ أَزِيدَ فِي الصَّلاَةِ قَالَ ‏ "‏ وَمَا ذَاكَ ‏"‏ ‏.‏ قَالُوا صَلَّيْتَ خَمْسًا ‏.‏ فَثَنَى رِجْلَهُ وَسَجَدَ سَجْدَتَيْنِ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 1254
In-book reference : Book 13, Hadith 76
English translation : Vol. 2, Book 13, Hadith 1255