Musannaf Ibn Abi Shaybah

Hazrat Saeed bin Musayyib narrates that Hazrat Bilal came to inform the Messenger of Allah (peace and blessings be upon him) about the time of prayer. He was told that the Messenger of Allah (peace and blessings be upon him) was sleeping. Hazrat Bilal then called out loudly: "As-salatu khayrun minan-nawm" (Prayer is better than sleep). After that, these words became a part of the adhan.