The Book of the Commencement of the Prayer
11/40. Chapter:
Making the two rak'ahs of Fajr brief
Sunan an-Nasai
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا جَرِيرٌ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عَمْرَةَ، عَنْ عَائِشَةَ، قَالَتْ إِنْ كُنْتُ لأَرَى رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي رَكْعَتَىِ الْفَجْرِ فَيُخَفِّفُهُمَا حَتَّى أَقُولَ أَقَرَأَ فِيهِمَا بِأُمِّ الْكِتَابِ
It was narrated that Aishah said: "I would see the Messenger of Allah (ﷺ) praying two rak'ahs of Fajr and making them so brief that I said: 'Did he recite the Umm Al-Kitab in them?'"
Reference : Sunan an-Nasa'i 946In-book reference : Book 11, Hadith 71English translation : Vol. 2, Book 11, Hadith 947