Musannaf Ibn Abi Shayba 13:3004

Abu Bakr narrated to us, he said: Shababa bin Sawwar narrated to us, from Shu'bah, from Qatadah, he said: I heard Rafi' Abu al-'Aliyah say: Ali said: "The judges are three: two are in the Fire and one is in Paradise." He mentioned the two who are in the Fire, and he said: "A man who acted unjustly intentionally, so he is in the Fire; and a man who sought the truth but erred, so he is in the Fire; and the last one sought the truth and attained it, so he is in Paradise." He said: So I said to Rafi': "Do you see this one who sought the truth but erred?" He said: "It was his duty, if he did not know the judgment, not to be a judge."