Sayyiduna Anas bin Malik (may Allah be pleased with him) says that Sirin wanted me to make a contract of manumission (mukatabah) with him, and I refused. He went to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) and mentioned it to him. Sayyiduna Umar (may Allah be pleased with him) came to me with the whip and said: Make the contract with him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21615