Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

"Adda bin Khalid bin Hawdhah said to me: 'Shall I not read to you a letter that the Messenger of Allah (saw), wrote to me?' I said: 'Yes.' So he took out a letter. In it was: 'This is what 'Adda' bin Khalid bin Hawdhah bought [from] Muhammad the Messenger of Allah (saw) He bought from him a slave' or 'a female slave, having no ailments, nor being a runaway, not having any malicious behavior. Sold by a Muslim to a Muslim.'"