It is narrated from Hafs bin Umar bin Sa'd al-Mu'adhdhin that Sayyiduna Sa'd (may Allah be pleased with him) was the muezzin appointed by the Prophet (peace and blessings be upon him). Hafs said: My family told me that Sayyiduna Bilal (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) to give the call for the morning prayer, and they said: He (peace and blessings be upon him) is asleep. Sayyiduna Bilal (may Allah be pleased with him) called out at the top of his voice: 'Al-salatu khayrun min al-nawm' (Prayer is better than sleep), and then it was made a fixed part of the morning prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1982