Musnad Ahmad 33:706

Yunus ibn Muhammad and Hujayn narrated to us, they said: Layth narrated to us, from Abu al-Zubayr, from Jabir, who said: We were one thousand four hundred on the day of al-Hudaybiyyah. We pledged allegiance to him while Umar was holding his hand under the tree, and it was a Samurah tree. He said: "We pledged allegiance to him that we would not flee, and we did not pledge allegiance to him upon death."