Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Harith that a person said, "I was the slave of such-and-such person, or my father was the slave of such-and-such person, or my mother was the bondwoman of such-and-such person." That person said, "Today you are my slave." The Prophet said that if they were already free and it is known that they are slaves, then they will not be considered slaves merely by someone’s claim, unless he brings just witnesses. And those witnesses testify that this person is a slave.