Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When the Messenger of Allah (peace and blessings be upon him) tied the banner for Sayyiduna Ali (may Allah be pleased with him) on the day of Khaybar, he supplicated for him for a while and said: "O Allah, help him and grant honour (to the religion) through him; have mercy on him and show mercy through him; support him and grant support through him. O Allah, befriend whoever befriends him, and be the enemy of whoever is his enemy."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12599