Musnad Ahmad 482:110

Abu al-Nadr narrated to us, al-Faraj narrated to us, Abdullah ibn Amir al-Aslami narrated to us, from Abu Ali al-Masri, who said: We traveled with Uqbah ibn Amir al-Juhani, and the time for prayer arrived, so we wanted him to lead us. We said: "You are from the companions of the Messenger of Allah, may Allah bless him and grant him peace, so do not let us go ahead of you." He said: "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Whoever leads a people in prayer, if he completes it, then for him is the completion and for them is the completion. And if he does not complete it, then for them is the completion, and upon him is the sin.'"