Hizam bin Hakim says that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) wrote a letter to Sayyiduna Umayr bin Sa'd (may Allah be pleased with him): Forbid the Muslims (under you) from making contracts of manumission (mukatabah) with their slaves on the basis of begging from people.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21619