Sunan Ibn Majah

It was narrated that Bilal came to the Prophet 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