Sa'id bin al-Musayyib said that Sayyiduna Bilal (may Allah be pleased with him) once came to the Prophet (peace and blessings be upon him) to inform him of the prayer, and he was told that he (peace and blessings be upon him) was sleeping. So he called out: "Prayer is better than sleep." Then this was made part of the Fajr adhan.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1074