Bukhari

Narrated Abu Sa`id Al-Khudri: Allah's Apostle said, "Whenever you hear the Adhan, say what the Mu'adh-dhin is saying.

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.)"

Narrated Yahya as above (586) and added: "Some of my companions told me that Hisham had said, "When the Mu'adh-dhin said, "Haiyi `alassala (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."