Musnad Ahmad 11:9

Muhammad ibn Abi ‘Adi narrated to us, from Dawud, from ‘Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent the army of Dhat al-Salasil. He appointed Abu ‘Ubaydah over the Muhajirun, and he appointed ‘Amr ibn al-‘As over the Bedouins. He said to both of them: "Obey one another." He said: They were ordered to raid [the tribe of] Bakr. So ‘Amr set out and raided [the tribe of] Quda‘ah, because Bakr were his maternal uncles. Al-Mughirah ibn Shu‘bah went to Abu ‘Ubaydah and said: "The Messenger of Allah (peace and blessings of Allah be upon him) appointed you over us, and the son of so-and-so has taken charge of the people’s affairs, and you have no authority with him." Abu ‘Ubaydah said: "The Messenger of Allah (peace and blessings of Allah be upon him) ordered us to obey one another. So I will obey the Messenger of Allah (peace and blessings of Allah be upon him), even if ‘Amr disobeys him."