صَحِيحُ مُسْلِمٍ
Sahih Muslim 724 f
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
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنِ ابْنِ جُرَيْجٍ، قَالَ حَدَّثَنِي عَطَاءٌ، عَنْ عُبَيْدِ بْنِ عُمَيْرٍ، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم لَمْ يَكُنْ عَلَى شَىْءٍ مِنَ النَّوَافِلِ أَشَدَّ مُعَاهَدَةً مِنْهُ عَلَى رَكْعَتَيْنِ قَبْلَ الصُّبْحِ .
'A'isha reported that the Messenger (ﷺ) was not so much particular about observing supererogatory rak'ahs as in case of the two rak'ahs of the dawn prayer.
Sahih Muslim, 724 fSahihصحيح