Musannaf Ibn Abi Shayba 17:1369

Abu Dawud al-Tayalisi narrated to us, from Shu'bah, who said: I was walking with al-Hakam, and we saw Abu Ma'shar. Al-Hakam said: "This man has heard something about me that I supposedly said, but by Allah, besides whom there is no god, I did not say it." He said: When Abu Ma'shar came, al-Hakam apologized to him and said: "I have sworn to Shu'bah that I did not say what reached you about me."