بُلُوغُ الْمَرَامِ
Bulugh al-Maram 363
The Book of Prayer
وَعَنْ عَائِشَةَ -رَضِيَ اَللَّهُ عَنْهَا- قَالَتْ : { كَانَ اَلنَّبِيُّ - صلى الله عليه وسلم -يُخَفِّفُ اَلرَّكْعَتَيْنِ اَللَّتَيْنِ قَبْلَ صَلَاةِ اَلصُّبْحِ , حَتَّى إِنِّي أَقُولُ : أَقَرَأَ بِأُمِّ اَلْكِتَابِ? } مُتَّفَقٌ عَلَيْهِ . [1]
Narrated 'Aishah (RA): The Prophet (ﷺ) used to make the two (voluntary) Rak'at before the Fajr prayer so short that I would wonder whether he recited al-Fatiha (or not). [Agreed upon].
Sunnah.com reference: Book 2, Hadith 267