Musnad Abu Ya'la al-Mawsili 3:307

Al-Qawariri narrated to us, Yunus bin Arqam narrated to us, Yazid bin Abi Ziyad narrated to us, from Abd al-Rahman bin Abi Layla, who said: I witnessed Ali in the Rahba adjuring the people. He said: "I adjure by Allah anyone who heard the Messenger of Allah (peace and blessings of Allah be upon him) say on the day of Ghadir Khumm, 'Whoever I am his master (mawla), then Ali is his master,' to stand and testify." Abd al-Rahman said: Twelve men who had participated in Badr stood up—it is as if I am looking at one of them wearing trousers—and they said: "We testify that we heard the Messenger of Allah (peace and blessings of Allah be upon him) say on the day of Ghadir Khumm: 'Am I not more worthy of the believers than their own selves, and my wives are their mothers?' We said: 'Yes, O Messenger of Allah.' He said: 'Then whoever I am his master, then Ali is his master. O Allah, befriend whoever befriends him and be hostile to whoever is hostile to him.'"