Sahih Muslim 724
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
'A'isha reported:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَابْنُ، نُمَيْرٍ جَمِيعًا عَنْ حَفْصِ بْنِ غِيَاثٍ ، - قَالَ ابْنُ نُمَيْرٍ حَدَّثَنَا حَفْصٌ ، - عَنِ ابْنِ جُرَيْجٍ ، عَنْ عَطَاءٍ ، عَنْ عُبَيْدِ بْنِ عُمَيْرٍ ، عَنْ عَائِشَةَ ، قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فِي شَىْءٍ مِنَ النَّوَافِلِ أَسْرَعَ مِنْهُ إِلَى الرَّكْعَتَيْنِ قَبْلَ الْفَجْرِ .
I have never seen the Messenger of Allah (ﷺ) hastening as much in observing supererogatory as two rak'ahs before the (Fard) of the dawn prayer.