Musnad Abu Ya'la al-Mawsili 117:13

Hadba and Hawthara narrated to us, they said: Hammad narrated to us, from Thabit, from Anas, who said: The people of Yemen came to the Messenger of Allah, may Allah bless him and grant him peace. They said: "Send a man with us to teach us." He said: So he took the hand of Abu Ubaydah and sent him with them, and he said: "This is the trustworthy one of this nation."