Musannaf Ibn Abi Shayba 13:218

Abu Bahr al-Bakrawi narrated to us, from Muhammad ibn Abi Yahya, who said: My mother informed me that her grandfather was a *mukātab* (a slave with a contract of manumission) to Abdullah ibn Qays al-Aslami. He wanted to depart for Basra, but he (Abdullah) prevented him. So he came to Uthman and said, "You do not have the right to prevent him." So he let him go.