Musnad Ahmad 5:109

Muhammad ibn Abdullah narrated to us, al-Rabi’—meaning ibn Abi Salih al-Aslami—narrated to me, Ziyad ibn Abi Ziyad narrated to me, [saying]: I heard Ali ibn Abi Talib adjuring the people, and he said: "I adjure by Allah any Muslim man who heard the Messenger of Allah, may Allah bless him and grant him peace, say on the day of Ghadir Khumm what he said." So twelve men who had participated in Badr stood up and testified.