Musannaf Ibn Abi Shayba 13:3034

Abu Bakr narrated to us, he said: Ibn Abi Za’ida narrated to us, from al-A’mash, from al-Qasim, from his father, from ‘Abdullah, the like of it, except that he added in it: "If a matter comes to him that he does not know, then let him confess, and let him not be ashamed."