Musnad Ahmad 5:386

'Abdullah narrated to us, 'Ali ibn Hakim al-Awdi narrated to us, Sharik informed us, from Abu Ishaq, from Sa'id ibn Wahb and from Zayd ibn Yuthay', they both said: 'Ali adjured the people at al-Rahbah: "Whoever heard the Messenger of Allah, peace and blessings of Allah be upon him, say on the day of Ghadir Khumm, let him stand." They said: So six men stood up from the side of Sa'id, and six men from the side of Zayd, and they testified that they heard the Messenger of Allah, peace and blessings of Allah be upon him, say to 'Ali on the day of Ghadir Khumm: "Is Allah not more worthy of the believers?" They said: "Yes." He said: "O Allah, whoever I am his master (mawla), then 'Ali is his master. O Allah, befriend whoever befriends him, and be hostile to whoever is hostile to him."