It is narrated from Sayyiduna Abu Rafi' (may Allah be pleased with him) that when the mu'adhdhin called the adhan, the Prophet (peace and blessings be upon him) would repeat the same words that he said, and when he said "Hayya 'ala al-salah (Come to the 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)."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 920