Narrated Abu Said Al-Khudri:
Allah’s Apostle said, “Whenever you hear the Adhan, say what the Mu’adhdhin is saying.
Narrated ‘Isa bin Talha:
that he had heard Mu’awiya repeating the words of Adhan up to “Wa ash-hadu Anna Muhammadan Rasulul-lah (and I testify that Muhammad is Allah’s Apostle.)”
Narrated Yahya as above (586) and added:
“Some of my companions told me that Hisham had said, “When the Muadhin said, “Haiya alas-sala(t) (come for the prayer).” Muawiya said, “La hawla wala quwata illa billah (There is neither might nor any power except with Allah)” and added, “We heard your Prophet saying the same.”
So, when the muadhin (caller to prayer) is pronouncing the Adhaan, you’re supposed to repeat what he says. But when he says ‘hayya alas-salah‘ and ‘hayya alal-falah‘, say: ‘la hawla wala quwwata illa billah‘.