Abu Bakr bin Khalid bin Urfutah narrates that he came to Sayyiduna Sa'd bin Malik (may Allah be pleased with him) and said: "I have heard that you people are being pressed in Kufa to revile Sayyiduna Ali (may Allah be pleased with him). Have you reviled him?" He replied: "Allah forbid! By the One in whose Hand is the soul of Sa'd, I heard the Prophet (peace and blessings be upon him) say something about Sayyiduna Ali (may Allah be pleased with him) such that, even if a saw were placed on the parting of my hair, I would never revile him."
Abu Ya'la recorded this narration, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14739