Musannaf Ibn Abi Shayba 13:217

Waki’ narrated to us, from Isma’il, from al-Sha’bi, regarding a man who stipulated upon his *mukattab* (slave under a contract of manumission) that he should not leave [the city]. He said: "He may leave." Waki’ said: Sufyan said: "He shall not leave except with the permission of his master."