Sahih al-Bukhari, Volume 1, Book 11, Hadith 586 (USC-MSA) ⓘ
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 Messenger.)"
حَدَّثَنَا مُعَاذُ بْنُ فَضَالَةَ، قَالَ حَدَّثَنَا هِشَامٌ، عَنْ يَحْيَى، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ بْنِ الْحَارِثِ، قَالَ حَدَّثَنِي عِيسَى بْنُ طَلْحَةَ، أَنَّهُ سَمِعَ مُعَاوِيَةَ، يَوْمًا فَقَالَ مِثْلَهُ إِلَى قَوْلِهِ " وَأَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ " .
also referenced under: Book 10, Hadith 10 (In-Book) & Hadith 612 (DarusSalam)