Musnad Ahmad 5:80

Ibn Numayr narrated to us, 'Abd al-Malik narrated to us, from Abu 'Abd al-Rahim al-Kindi, from Zadhan Abu 'Amr, who said: I heard 'Ali in the Rahbah, while he was asking the people: "Who witnessed the Messenger of Allah, peace and blessings of Allah be upon him, on the day of Ghadir Khumm, while he was saying what he said?" Thirteen men stood up and testified that they heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever I am his master (mawla), then 'Ali is his master (mawla)."