Musnad Ahmad 32:1274

'Abd al-Samad narrated to us, Hammad narrated to us, from Thabit, from Anas, who said: When the people of Yemen came to the Prophet, may Allah bless him and grant him peace, they said: "Send with us a man to teach us the Book of our Lord and the Sunnah." He said: So the Prophet, may Allah bless him and grant him peace, took the hand of Abu 'Ubaydah and handed him over to them and said: "This is the trustworthy one of this nation."