The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
20/57. Chapter:
The time for the two rak'ahs of Fajr
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنْ حَفْصَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ كَانَ إِذَا نُودِيَ لِصَلاَةِ الصُّبْحِ رَكَعَ رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ يَقُومَ إِلَى الصَّلاَةِ .
It was narrated from Hafsah that: When the call for Subh prayer was given, the Messenger of Allah (ﷺ) would pray two brief rak'ahs before going to the prayer.
Reference : Sunan an-Nasa'i 1760In-book reference : Book 20, Hadith 163English translation : Vol. 2, Book 20, Hadith 1761