Musnad Ahmad 32:846

Mu'ammal narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, that a delegation from the people of Yemen came to the Prophet, may Allah bless him and grant him peace. He wanted to send a man with them, so they said, "Send a man with us." He said, "I will send with you the trustworthy one of this nation." So he sent Abu Ubaidah ibn al-Jarrah. In another instance, they said, "O Messenger of Allah, send a man with us to teach us." The Messenger of Allah, may Allah bless him and grant him peace, took the hand of Abu Ubaidah ibn al-Jarrah and said, "For every nation there is a trustworthy one, and this is the trustworthy one of this nation."