Musannaf Ibn Abi Shayba 2:252

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-Hasan, from Uthman bin Abi al-As, he said: The last thing the Prophet, may Allah bless him and grant him peace, enjoined upon us was that a mu'adhdhin should not take a wage for his adhan.