Musnad Ahmad 658:12

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Ali ibn Zayd informed us, from 'Adi ibn Thabit, from al-Bara' ibn 'Azib, who said: We were with the Messenger of Allah, may Allah bless him and grant him peace, on a journey, and we stopped at Ghadir Khumm. It was announced among us: "Prayer is gathering." The ground under two trees was swept for the Messenger of Allah, may Allah bless him and grant him peace. He prayed the noon prayer, then took the hand of 'Ali, may Allah be pleased with him, and said: "Do you not know that I am closer to the believers than their own selves?" They said: "Yes." He said: "Do you not know that I am closer to every believer than his own self?" They said: "Yes." He said: "So he took the hand of 'Ali and said: '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.'" He said: Then 'Umar met him after that and said: "Congratulations to you, O son of Abu Talib! You have become, morning and evening, the master of every believing man and believing woman."