Sahih Muslim
Sahih Muslim 727 c
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
Arabic hadith text
وَحَدَّثَنِي عَلِيُّ بْنُ خَشْرَمٍ، أَخْبَرَنَا عِيسَى بْنُ يُونُسَ، عَنْ عُثْمَانَ بْنِ حَكِيمٍ، فِي هَذَا الإِسْنَادِ . بِمِثْلِ حَدِيثِ مَرْوَانَ الْفَزَارِيِّ
Translation
This hadith has been transmitted by another chain of narrators.