Sahih Muslim 723
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is recommended to pray two rak`ah for the sunnah of Fajr. And encouragement to pray them regularly, and to make them brief, and to persist in offering them, and clarifying what is recommended to recite therein
Hafsa reported:
حَدَّثَنَا مُحَمَّدُ بْنُ عَبَّادٍ ، حَدَّثَنَا سُفْيَانُ ، عَنْ عَمْرٍو ، عَنِ الزُّهْرِيِّ ، عَنْ سَالِمٍ ، عَنْ أَبِيهِ ، أَخْبَرَتْنِي حَفْصَةُ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ إِذَا أَضَاءَ لَهُ الْفَجْرُ صَلَّى رَكْعَتَيْنِ .
When the dawn appeared, the Messenger of Allah (ﷺ) observed two rak'ahs (of Sunnah prayers).