Musannaf Ibn Abi Shaybah 2181 — Those who said in the adhan: Prayer

Sa'id bin al-Musayyib said: Sayyiduna Bilal (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) to inform him of the time of prayer. He was told that the Prophet (peace and blessings be upon him) was asleep. Sayyiduna Bilal (may Allah be pleased with him) called out loudly: 'al-salatu khayrun min al-nawm (prayer is better than sleep)'. After that these words became part of the adhan.