Musannaf Ibn Abi Shayba 2:126

Yazid ibn Harun narrated to us, from Hajjaj, from a sheikh from the people of Medina, from some of the descendants of the muezzins of the Prophet, may God bless him and grant him peace, who said: Ibn Umm Maktum used to give the adhan (call to prayer) and Bilal would give the iqamah (call to commence prayer). And sometimes Bilal would give the adhan and Ibn Umm Maktum would give the iqamah.