Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1776 (DarusSalam) ⓘ
It was narrated from Abdullah bin Umar, from Hafsah, that: The Messenger of Allah used to pray two rak'ahs when dawn had broken.
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ يَزِيدَ، قَالَ حَدَّثَنَا أَبِي قَالَ، حَدَّثَنَا جُوَيْرِيَةُ بْنُ أَسْمَاءَ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ حَفْصَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يُصَلِّي رَكْعَتَيْنِ إِذَا طَلَعَ الْفَجْرُ .
also referenced under: Hadith 1775 (Reference) & Book 20, Hadith 178 (In-Book)