Musannaf Ibn Abi Shayba 2:41

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from ‘Imran ibn Muslim, from Suwayd ibn Ghaflah, that he used to send word to his mu’adhdhin: "When you reach 'Hayya 'ala al-falah' (Come to success), say: 'As-salatu khayrun min an-nawm' (Prayer is better than sleep)." For it is the adhan of Bilal.