Hazrat Ali (may Allah be pleased with him) said that if a free person admits to being a slave, he will be considered a slave.

Hazrat Sha'bi states that if a free person admits to being a slave over himself, he will not become a slave.

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.