Musannaf Ibn Abi Shayba 2:193

Abu Bakr narrated to us, he said: Muhammad ibn Bishr and Ibn Numayr narrated to us, from Ubayd Allah ibn Umar, from Nafi’, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) had two mu'adhdhins who would call the adhan. Ibn Numayr added in it: Ibn Umm Maktum and Bilal.