Narrated Abu Said Al-Khudri: Allah’s Apostle SALLALLAHU ‘ALAYHI WASALLAM said, “Whenever you hear the Adhan, say what the Mu’adhdhin is saying.(Volume 1, Book 11, Number 585) Narrated ‘Isa bin Talha: that he had heard Muawiya repeating the words of Adhan up to “Wa ash-hadu Anna Muhammadan Rasulul-lah (and I testify that Muhammad is Allah’s Apostle.)” (Volume 1, Book 11, Number 586:)

Narrated Yahya as above (586) and added:

“Some of my companions told me that Hisham had said, “When the Mu’adhdhin 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.”(Volume 1, Book 11, Number 587)

So While listening to the adhan, We Muslims should repeat the same words silently, except when the muezzin says ḥayya ‘alas-salāh or ḥayya ‘alal-falāḥ where we should silently say lā hawla wa lā quwata illā billāh (there is no strength or power except from God).