Musannaf Ibn Abi Shayba 21:654

'Isa ibn Yunus narrated to us, from al-Awza'i, from the one who informed him, from Mujahid, that when the mu'adhdhin would say, "Hayya 'ala al-salah" (Come to prayer), he would say, "Al-musta'anu billah" (The One whose help is sought is Allah). And when he would say, "Hayya 'ala al-falah" (Come to success), he would say, "La hawla wa la quwwata illa billah" (There is no power and no strength except by Allah).