The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
20/60. Chapter:
The time for the two rak'ahs of Fajr, and mentioning the differences reported from Nafi'
Sunan an-Nasai
أَخْبَرَنَا الْحُسَيْنُ بْنُ عِيسَى، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، عَنِ الزُّهْرِيِّ، عَنْ سَالِمٍ، عَنْ أَبِيهِ، قَالَ أَخْبَرَتْنِي حَفْصَةُ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا أَضَاءَ لَهُ الْفَجْرُ صَلَّى رَكْعَتَيْنِ .
It was narrated from Salim that his father said: "Hafsah told me that when dawn glowed, the Messenger of Allah (ﷺ) would pray two rak'ahs."
Reference : Sunan an-Nasa'i 1779In-book reference : Book 20, Hadith 182English translation : Vol. 2, Book 20, Hadith 1780