Musannaf Ibn Abi Shayba 28:601

'Abd al-Rahim ibn Sulayman narrated to us, from 'Ubayd Allah, from Nafi', from Aslam, the freed slave of 'Umar: That 'Umar wrote to his governors, forbidding them from killing women and children. And he commanded them to kill those upon whom the razors have run.