Musannaf Ibn Abi Shayba 13:1969

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from ‘Amr, that a man said to his slave: "Stick to so-and-so, for if you leave him, you are free." He said: "Bear witness that I have left him." That was raised to ‘Umar ibn ‘Abd al-‘Aziz while he was the governor of Makkah, and he validated his manumission. He said: Al-Hasan used to hold that view.