Musannaf Ibn Abi Shayba 34:301

Maslama narrated to us, Abu Bakr Muhammad ibn Ahmad ibn al-Jahm narrated to us in Baghdad, Abdullah ibn Ahmad ibn Muhammad ibn Hanbal narrated to us, he said: My father narrated to me, Hajjaj narrated to us, he said: I heard Abu Isra'il say: The first day I knew al-Hakam was the day al-Sha'bi died. He said: A person came asking about an issue, and they said to him: "You must go to al-Hakam ibn Utaybah."