Musnad Ahmad 32:321

Yazid ibn Harun narrated to us, Hammad ibn Salamah informed us, from Thabit al-Bunani, from Anas ibn Malik, that when the people of Yemen came to the Messenger of Allah, may Allah bless him and grant him peace, they asked him to send a man with them to teach them. So he sent Abu Ubaidah with them and said, "He is the trustworthy one of this nation."