Sahih Muslim 725
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 that the Messenger of Allah (ﷺ) said about the two (supererogatory) rak'ahs of the dawn:
وَحَدَّثَنَا يَحْيَى بْنُ حَبِيْبٍ ، حَدَّثَنَا مُعْتَمِرٌ ، قَالَ قَالَ أَبِي حَدَّثَنَا قَتَادَةُ ، عَنْ زُرَارَةَ ، عَنْ سَعْدِ بْنِ هِشَامٍ ، عَنْ عَائِشَةَ ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ فِي شَأْنِ الرَّكْعَتَيْنِ عِنْدَ طُلُوعِ الْفَجْرِ " لَهُمَا أَحَبُّ إِلَىَّ مِنَ الدُّنْيَا جَمِيعًا " .
They are dearer to me than the whole world.