Messenger of Allah ﷺ said, "Whenever you hear the Adhan, say what the Mu'adh-dhin is saying.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا سَمِعْتُمُ النِّدَاءَ فَقُولُوا مِثْلَ ما يَقُولُ الْمُؤَذِّنُ  

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 Messenger of Allah ﷺ.)"  

مِثْلَهُ إِلَى قَوْلِهِ وَأَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ  

"Some of my companions told me that Hisham had said, "When the Mu'adh-dhin said, "Haiyi ʿalassala (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."  

لاَ حَوْلَ وَلاَ قُوَّةَ إِلاَّ بِاللَّهِ وَقَالَ هَكَذَا سَمِعْنَا نَبِيَّكُمْ ﷺ يَقُولُ