Musannaf Ibn Abi Shayba 27:484

Ja'far ibn 'Awn narrated to us, he said: Sufyan ibn Abi 'Abd Allah narrated to us, he said: Abu Bakr ibn Khalid ibn 'Arfata narrated to us, he said: I came to Sa'd ibn Malik in Medina. He said: "It has been mentioned to me that you all insult 'Ali." He said: "We have indeed done so." He said: "Perhaps you have insulted him?" He said: "I said: 'I seek refuge in Allah.'" He said: "Then do not insult him. For if a saw were placed upon the crown of my head [to force me] to insult 'Ali, I would never insult him after what I heard from the Messenger of Allah (may Allah bless him and grant him peace) of what I heard."