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
Sa'id bin al-Musayyib narrates from Sayyiduna Bilal (may Allah be pleased with him); Bilal (may Allah be pleased with him) said: "We were with the Prophet (peace and blessings be upon him) on a journey, and the people slept until the sun rose. So he (peace and blessings be upon him) ordered Bilal, and he gave the adhan. Then they prayed the two (sunnah) rak'ahs of Fajr, then they prayed the (obligatory) Fajr prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1075