Musnad Abu Ya'la al-Mawsili 243:18

Ishaq ibn Ibrahim al-Harawi narrated to us, Hushaym narrated to us, from Abd al-Rahman ibn Ishaq, from Abu Burdah ibn Abi Musa al-Ash'ari, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been given the beginnings of speech and its endings." We said: "O Messenger of Allah, teach us from what Allah, the Mighty and Majestic, has taught you." So he taught us the Tashahhud.