Sunan Ibn Majah, Volume 1, Book 3, Hadith 714 (DarusSalam) ⓘ
It was narrated that 'Uthman bin Abul-As said: "The last instruction that the Messenger of Allah gave to me was that I should not appoint a Mu'adh-dhin who took payment for the Adhan." (sahih)
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حَفْصُ بْنُ غِيَاثٍ، عَنْ أَشْعَثَ، عَنِ الْحَسَنِ، عَنْ عُثْمَانَ بْنِ أَبِي الْعَاصِ، قَالَ كَانَ آخِرُ مَا عَهِدَ إِلَىَّ النَّبِيُّ ـ صلى الله عليه وسلم ـ أَنْ لاَ أَتَّخِذَ مُؤَذِّنًا يَأْخُذُ عَلَى الأَذَانِ أَجْرًا .
also referenced under: Hadith 714 (Reference) & Book 3, Hadith 9 (In-Book)