Musnad Ahmad 1302:18

Zayd ibn al-Hubab narrated to us, Mu'awiyah ibn Salih narrated to us, Abu al-Zahiriyah Hudayr ibn Kurayb narrated to me, from Kathir ibn Murrah al-Hadrami, who said: I heard Abu al-Darda' saying: The Messenger of Allah (peace and blessings of Allah be upon him) was asked: "Is there recitation in every prayer?" He said: "Yes." A man from the Ansar said: "This has become obligatory." Abu al-Darda' turned to me, and I was the closest of the people to him, and said: "O son of my brother, I do not see the Imam, when he leads the people, but that he has sufficed them."