Musnad Ahmad 178:1

Muhammad ibn Ja‘far narrated to us, ‘Awf narrated to us, he said: ‘Alqamah al-Muzani narrated to me, he said: A man narrated to me, he said: I was in a gathering in which ‘Umar ibn al-Khattab was present in Medina. He said to a man from the people: "O so-and-so, how did you hear the Messenger of Allah, peace and blessings of Allah be upon him, describe Islam?" He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Indeed, Islam began as a *jadha’* (a one-year-old camel), then a *thani* (a two-year-old camel), then a *ruba‘i* (a four-year-old camel), then a *sudais* (a six-year-old camel), then a *bazil* (a fully mature camel)." ‘Umar ibn al-Khattab said: "And there is nothing after the *bazil* stage except decline."