كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(14)
Chapter: 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
(14)
باب اسْتِحْبَابِ رَكْعَتَيْ سُنَّةِ الْفَجْرِ وَالْحَثِّ عَلَيْهِمَا وَتَخْفِيفِهِمَا وَالْمُحَافَظَةِ عَلَيْهِمَا وَبَيَانِ مَا يُسْتَحَبُّ أَنْ يُقْرَأَ فِيهِمَا
Sahih Muslim 727 c

This hadith has been transmitted by another chain of narrators.

وَحَدَّثَنِي عَلِيُّ بْنُ خَشْرَمٍ، أَخْبَرَنَا عِيسَى بْنُ يُونُسَ، عَنْ عُثْمَانَ بْنِ حَكِيمٍ، فِي هَذَا الإِسْنَادِ ‏.‏ بِمِثْلِ حَدِيثِ مَرْوَانَ الْفَزَارِيِّ ‏.‏
Reference : Sahih Muslim 727c
In-book reference : Book 6, Hadith 123
USC-MSA web (English) reference : Book 4, Hadith 1578
  (deprecated numbering scheme)