Sa'id bin al-Musayyib (may Allah have mercy on him), relating the story of the dream of Abdullah bin Zayd (may Allah be pleased with him), said: Then Bilal (may Allah be pleased with him) added these words to the adhan: 'Al-salatu khayrun min al-nawm' (Prayer is better than sleep). Bilal (may Allah be pleased with him) came, after giving the adhan for the Fajr prayer, to inform the Prophet (peace and blessings be upon him) of the prayer, and he was told that the Prophet (peace and blessings be upon him) was asleep. 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 adhan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1983