Musannaf Ibn Abi Shayba 27:501

Muttalib ibn Ziyad narrated to us, from Layth, who said: I entered upon Abu Ja’far, and he mentioned his sins and what he fears. He said: Then he wept, and then said: Jabir narrated to me that Ali carried the gate on the day of Khaybar until the Muslims ascended and opened it, and that it was tested and forty men could not carry it.