(60) باب وَقْتِ رَكْعَتَىِ الْفَجْرِ وَذِكْرِ الاِخْتِلاَفِ عَلَى نَافِعٍ
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 1779
In-book reference: Book 20, Hadith 182
English translation: Vol. 2, Book 20, Hadith 1780