صَحِيحُ مُسْلِمٍ
Sahih Muslim 724 a
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
حَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، حَدَّثَنَا هِشَامُ بْنُ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي رَكْعَتَىِ الْفَجْرِ إِذَا سَمِعَ الأَذَانَ وَيُخَفِّفُهُمَا .
'A'isha reported: The Messenger of Allah (ﷺ) used to observe two rak'ahs of Sunnah (prayer) when he heard the Adhin and shortened them.
Sahih Muslim, 724 aSahihصحيح