Sayyiduna Abu Mahdhurah (may Allah be pleased with him) says: I used to give the adhan for the Messenger of Allah (peace and blessings be upon him) for the Fajr prayer, and in the first adhan, when I had said "Hayya 'ala al-falah," I would say: "Al-salatu khayrun min al-nawm, al-salatu khayrun min al-nawm (Prayer is better than sleep)."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6708