Musnad Ahmad 33:216

Nasr ibn Bab narrated to us, from Hajjaj, from al-Dhayyal ibn Harmalah, who said: I asked Jabir ibn Abdullah al-Ansari, "How many were you on the day of the tree?" He said: "We were one thousand four hundred." He said: "And the Messenger of Allah, peace and blessings of Allah be upon him, used to raise his hands in every takbir of the prayer."