It is narrated from Sayyiduna Bilal (may Allah be pleased with him) that he came to the Prophet (peace and blessings be upon him) to inform him of Fajr and found him (peace and blessings be upon him) asleep. So he said twice: Prayer is better than sleep. The Prophet (peace and blessings be upon him) said: "How excellent this is, O Bilal! Include it in your adhan."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1077