I used to see the Messenger of Allah ﷺ pray the two units before Fajr so briefly that I would wonder whether he had even recited the Opening of the Book in them.
أخبرنا إسحق بن إبراهيم قال أنبأنا جرير عن يحيى بن سعيد عن محمد بن عبد الرحمن عن عمرة
إن كنت لأرى رسول الله ﷺ يصلي ركعتي الفجر فيخففهما حتى أقول أقرأ فيهما بأم الكتاب
No references are available.
Bukhari, Tahajjud, chapter on what is recited in the two rakahs of Fajr, no. 1171, and Muslim, Prayer of the Traveller, chapter on the recommendation of the two sunnah rakahs of Fajr, etc, no. 92/724, from the hadith of Yahya bin Saeed Ansari, in this manner. It is also in Kubra, no. 1018.