Sayyiduna Ali (may Allah be pleased with him) narrates: On the Day of Ghadir Khumm the Prophet (peace and blessings be upon him) said: "Whoever I am the mawla of, Ali is also his mawla." The narrator says: The narrators added these words after that: "Befriend whoever befriends him, and be hostile to whoever is hostile to him."
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14628