The narrator relates: I heard Sayyiduna Mu'awiyah (may Allah be pleased with him) say: I heard the Messenger of Allah (peace and blessings be upon him) say many times that when the muezzin called the adhan and pronounced the testimony, the Messenger of Allah (peace and blessings be upon him) would also repeat the words of the testimony just as the muezzin said them; then when he said "Hayya 'ala al-salah (Come to prayer)", he (peace and blessings be upon him) would say: "La hawla wa la quwwata illa billah (There is no might and no power except with Allah)"; and when he said "Hayya 'ala al-falah (Come to success)", he (peace and blessings be upon him) would again say the same: "La hawla wa la quwwata illa billah."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16976