Musannaf Ibn Abi Shayba 13:1118

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Isma'il ibn Salim, from al-Sha'bi, who said: I heard him narrating that a man bought a slave boy from a man. When he departed with him, behold, he had ringworm (tinea capitis). So he disputed with his companion before Shurayh. He said: He said, "I bought this slave boy from this man, and he has ringworm. So look at his ringworm, for ringworm does not happen suddenly." Shurayh said, "I will not combine being a judge and a witness. Show him to someone other than me, then come to me with them, and let them testify for you. Otherwise, his oath by Allah that he did not sell him to you while he had this ringworm."