Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 665
Narrated Anas:
The Prophet said, "Every nation has an Amin (i.e. the most honest man), and the Amin of this nation is Abu 'Ubaida bin Al-Jarrah."