Ada' bin Khalid bin Hawdhah (may Allah be pleased with him) said: "Shall I not read to you the letter that the Messenger of Allah (peace and blessings be upon him) wrote for me?" We said: Certainly. In it was written: "In the name of Allah, the Most Gracious, the Most Merciful. This is what Ada' bin Khalid bin Hawdhah bought from Muhammad, the Messenger of Allah (peace and blessings be upon him); he bought from him a male or a female slave." Uthman was unsure. "It is the sale of a Muslim to a Muslim; there is no disease, deceit or defect in it, nor anything unlawful or forbidden."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10783