It is narrated from Sayyiduna Abu Mahdhurah (may Allah be pleased with him): I was the muezzin appointed by the Prophet (peace and blessings be upon him), and in the first adhan of Fajr, after 'Hayya 'ala al-falah, hayya 'ala al-falah' (Come to success, come to success), I used to say: "'Al-salatu khayrun min al-nawm, al-salatu khayrun min al-nawm, Allahu akbar, Allahu akbar, la ilaha illallah' (Prayer is better than sleep, prayer is better than sleep, Allah is the Greatest, Allah is the Greatest, there is no god but Allah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1981