Musnad Ahmad 32:540

Hassan narrated to us, Hammad ibn Salamah narrated to us, from Thabit al-Bunani, from Anas ibn Malik, that the people of Yemen came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "Send with us a man who will teach us." The Messenger of Allah, may Allah bless him and grant him peace, took the hand of Abu Ubaidah ibn al-Jarrah and sent him with them, saying: "This is the trustworthy one of this nation."