Sunan An-Nasai, Chapter: The Book of the Adhan (The Call to Prayer)

"I was with Muawiyah when the Muadhdhin called the Adhan. Mu'awiyah said what the Mu'adhdhin said, but when he said: 'Hayya 'alas-salah (come to prayer),' he said: 'La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah);' and when he said: 'Hayya 'alal-falah (come to prosperity),' he said: 'La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah).' After that he said what the Muadhdhin said, then he said: 'I heard the Messenger of Allah (saw) saying exactly like that.'" (Sahih)