The Messenger of Allah ﷺ was not more diligent in observing any voluntary prayer than he was with the two units before the dawn prayer.
حدثنا مسدد حدثنا يحيى عن ابن جريج حدثني عطاء عن عبيد بن عمير
إن رسول الله ﷺ لم يكن على شيء من النوافل أشد معاهدة منه على الركعتين قبل الصبح
Bukhari narrated it in tahajjud, chapter on keeping to the two rakahs of Fajr and whoever called them a voluntary prayer, hadith no. 1169. And Muslim, in the prayer of the traveler, chapter on the recommendation of the two rakahs of the sunnah of Fajr and encouragement to perform them, etc., hadith no. 724, from the hadith of Yahya, with the same wording, Qattan, 94.