Sunan an-Nasa'i, Hadith 946 ⓘ
It was narrated that Aishah said: "I would see the Messenger of Allah praying two rak'ahs of Fajr and making them so brief that I said: 'Did he recite the Umm Al-Kitab in them?'"
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
also referenced under: Book 11, Hadith 71 (In-Book) & Volume 2, Book 11, Hadith 947 (DarusSalam)