Musannaf Ibn Abi Shayba 3:2481

Ibn Adam narrated to us, from Isra'il, from Bayan, who said: Qays ibn Abi Hazim used to lead us in prayer. The mu'adhdhin (caller to prayer) established the prayer while he (Qays) had already prayed one rak'ah. He said: So he abandoned it, then stepped forward and led us in prayer.