Musannaf Ibn Abi Shaybah 21382 — A mukatab whose master stipulates

Muhammad bin Abi Yahya said: My mother told me that her grandfather was the mukatab of Abdullah bin Qays al-Aslami. He intended to go to Basrah, but Abdullah bin Qays forbade him. Her grandfather came to Sayyiduna Uthman (may Allah be pleased with him) and asked about the issue, and he said: You may not forbid him. So he was allowed to go.