It is narrated from Sayyiduna Umar (may Allah be pleased with him) that he said to the muezzin: When you reach 'Hayya 'ala al-falah' (Come to success) in Fajr, say: 'Al-salatu khayrun min al-nawm, al-salatu khayrun min al-nawm' (Prayer is better than sleep, prayer is better than sleep).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1987