صَحِيحُ مُسْلِمٍ
Sahih Muslim 724 d
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
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الْوَهَّابِ، قَالَ سَمِعْتُ يَحْيَى بْنَ سَعِيدٍ، قَالَ أَخْبَرَنِي مُحَمَّدُ بْنُ عَبْدِ الرَّحْمَنِ، أَنَّهُ سَمِعَ عَمْرَةَ، تُحَدِّثُ عَنْ عَائِشَةَ، أَنَّهَا كَانَتْ تَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي رَكْعَتَىِ الْفَجْرِ فَيُخَفِّفُ حَتَّى إِنِّي أَقُولُ هَلْ قَرَأَ فِيهِمَا بِأُمِّ الْقُرْآنِ .
'A'isha reported that the Messenger of Allah (ﷺ) observed two rak'ahs of the dawn prayer and he shortened them (to the extent) that I (out of surprise) said: Did he recite in them Surah Fatiha (only)?
Sahih Muslim, 724 dSahihصحيح