Musannaf Ibn Abi Shayba 3:1371

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Al-Shaybani informed us, from Khawat ibn Jubayr, he said: Yazid ibn Sharik al-Taymi, Abu Ibrahim al-Taymi, narrated to us, he said: I asked Umar ibn al-Khattab about reciting behind the Imam. He said to me: "Recite." I said: "Even if I am behind you?" He said: "Even if you are behind me." He said: "Even if you recite."