Musannaf Ibn Abi Shayba 18:1188

Abu Bakr narrated to us, he said: Abda narrated to us, from Ubayd Allah ibn Umar, from Nafi, from Ibn Umar, that a slave girl of Hafsa bewitched her, and they found her sorcery, and she confessed to it. So she (Hafsa) ordered Abd al-Rahman ibn Zayd, and he killed her. That reached Uthman, and he disapproved of it and it was severe upon him. Then Ibn Umar came to him and informed him that she had bewitched her, and she had confessed to it, and they had found her sorcery. It was as if Uthman only disapproved of that because she was killed without his permission.