سنن ابن ماجه

It was narrated that Bilal came to the Prophet (ﷺ) to call him for the Fajr prayer, and was told: "He is sleeping." He said: "As-salatu khairum minan-nawm, As-salatu khairum minan-nawm (The prayer is better than sleep, the prayer is better than sleep). These words were approved of in the Adhan for the Fajr, and that is how it remained.