Musnad Ahmad 33:1142

Sulayman ibn Dawud narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Musa ibn Uqbah, from Abu al-Zubayr, from Jabir, who said: Al-Abbas was holding the hand of the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, was making a covenant with us. When we finished, the Messenger of Allah, may Allah bless him and grant him peace, said: "I have taken, and I have given." He said: So I asked Jabir on that day, "How did you pledge allegiance to the Messenger of Allah, may Allah bless him and grant him peace? Was it upon death?" He said: "No, but we pledged allegiance to him that we would not flee." I said to him: "What about the Day of the Tree?" He said: "I was holding the hand of Umar ibn al-Khattab until we pledged allegiance to him." I said: "How many were you?" He said: "We were fourteen hundred. We all pledged allegiance to him except al-Jadd ibn Qays, who hid under the belly of a camel. And we slaughtered on that day seventy camels, one for every seven people."