Hisnul Muslim 22

Upon hearing the athan

لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللّهِ

Laa hawla wa laa quwwata 'illaa billaah.

Repeat what the muazzin says, except for when he says: حَيَّ عَلَى الصَّلاَةِ وَحَيَّ عَلَى الْفَلَاحِ.Here you should says: "There is no might and no power except by Allah".