(3) باب السُّنَّةِ فِي الأَذَانِ
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.
Reference: Sunan Ibn Majah 716
In-book reference: Book 3, Hadith 11
English translation: Vol. 1, Book 3, Hadith 716