Musannaf Ibn Abi Shayba 28:257

'Abd al-Rahim ibn Sulayman narrated to us, from 'Ubaydah, from Ibrahim, regarding a man who commits a crime, then joins the enemy and they grant him safety. He said: They are granted safety, unless a specific thing is identified, in which case it is taken from them and returned to its owners. As for him, he is held accountable for what he had committed before he joined them.