Musannaf Ibn Abi Shayba 2:246

Abu Bakr narrated to us, he said: Isa ibn Yunus narrated to us, from al-Awza’i, from the one who informed him, from Mujahid: That when the mu'adhdhin said, "Hayya 'ala al-salah" (Come to prayer), he would say, "Al-musta'an Allah" (Allah is the One whose help is sought). And when he said, "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).