Humayd bin Umarah narrates: I heard my father say: I heard the Prophet (peace and blessings be upon him) say, while holding the hand of Sayyiduna Ali (may Allah be pleased with him): "Whoever I am the mawla of, this man is also his mawla. O Allah, befriend whoever befriends him, and be hostile to whoever is hostile to him."
Al-Bazzar recorded it, and I do not know Humayd. The rest of its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14631