Musnad Ahmad 32:1023

Isma'il narrated to us, Khalid al-Hadhdha' narrated to us, from Abu Qilabah, who said: Anas said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, every nation has a trustee, and our trustee, O nation, is Abu Ubaidah ibn al-Jarrah."