Sayyiduna al-Harith bin Nawfal (may Allah be pleased with him) said: Whenever the Prophet (peace and blessings be upon him) heard the mu'adhdhin, he would say what he said, and when he said: Hayya 'ala al-salah, hayya 'ala al-falah, he (peace and blessings be upon him) would say: "La hawla wa la quwwata illa billah" (There is no power and no strength except with the help of Allah).
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3253