Musnad Abu Ya'la al-Mawsili 2:53

Abu Sa'id al-Qawariri narrated to us, Yazid bin Zuray' and Yahya bin Sa'id narrated to us, they said: 'Awf narrated to us, he said: 'Alqama bin Abdullah al-Muzani narrated to me—Yazid said in his hadith: "in the mosque of Basra"—he said: A man narrated to me, whom he had named, but 'Awf forgot his name. Yahya said: A man narrated to me, he said: I was in Medina in a gathering in which Umar bin al-Khattab was present. He said to some of those sitting with him: "How did you hear the Messenger of Allah, may Allah bless him and grant him peace, describe Islam?" He said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Indeed, Islam began as a young camel (jadha'), then as a two-year-old (thani), then as a four-year-old (ruba'i), then as a six-year-old (sudais), then as a fully mature camel (bazil)." Umar said: "And there is nothing after full maturity (bazul) except decline."