Sunan an-Nasa'i, Hadith 1779 ⓘ
It was narrated from Salim that his father said: "Hafsah told me that when dawn glowed, the Messenger of Allah would pray two rak'ahs."
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 .
also referenced under: Book 20, Hadith 182 (In-Book) & Volume 2, Book 20, Hadith 1780 (DarusSalam)