Musnad Ahmad 32:1619

'Affan narrated to us, Shu'bah narrated to us, he said: Khalid al-Hadhdha' informed me, from Abu Qilabah, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, for every nation there is a trustee, and the trustee of this nation is Abu 'Ubaydah ibn al-Jarrah."