Musnad Abu Ya'la al-Mawsili 6:89

Abu Khaythama narrated to us, Ubayd Allah ibn Musa narrated to us, Shaqiq ibn Abi Abd Allah informed us, from Abu Bakr ibn Khalid ibn Arfata, that he came to Sa’d ibn Malik and said: "It has reached me that you are being presented with the cursing of Ali in Kufa, so have you cursed him?" He said: "I seek refuge in Allah." He said: "By the One in whose hand is the soul of Sa’d, I heard the Messenger of Allah, may Allah bless him and grant him peace, say something about Ali; if a saw were placed upon my head [to force me] to curse him, I would never curse him."