Musnad Ahmad 5:743

Abdullah narrated to us, Hajjaj ibn al-Sha'ir narrated to me, Shababa narrated to us, Nu'aym ibn Hakim narrated to me, Abu Maryam and a man from the companions of Ali narrated to me, from Ali: That the Prophet, peace and blessings of Allah be upon him, said on the day of Ghadir Khumm: "Whoever I am his master (mawla), then Ali is his master (mawla)." He said: Then the people added after that: "And befriend whoever befriends him, and be hostile to whoever is hostile to him."