Abdullah bin Abi Bakr bin Anas (may Allah have mercy on him) says: The contract of manumission (mukatabah) of Sirin is with us: this is the contract that Anas bin Malik (may Allah be pleased with him) made with his slave Sirin, for so many thousand and two slaves, who would do the same kind of work (as he did).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21630