Musannaf Ibn Abi Shayba 27:434

Muttalib ibn Ziyad narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Jabir ibn Abdullah, who said: We were at al-Juhfa, at Ghadir Khumm, when the Messenger of Allah (may Allah bless him and grant him peace) came out to us. He took the hand of Ali and said: "Whoever I was his master (mawla), then Ali is his master (mawla)."