Musnad Ahmad 1018:46

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Abu Ishaq, who said: I heard Sa‘id ibn Wahb say: ‘Ali adjured the people, so five or six of the companions of the Prophet, may Allah bless him and grant him peace, stood up and testified that the Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever I am his master (mawla), then ‘Ali is his master (mawla)."