Musannaf Ibn Abi Shayba 13:3096

Abu Bakr narrated to us, he said: Abd al-Salam ibn Harb narrated to us, from Hajjaj, from Amr ibn Shu'ayb, from Sa'id ibn al-Musayyib; and from Hajjaj, from al-Hakam and Ata, regarding a man who says to his slave: "If you enter the house, you are free," then he sells him, and he (the slave) enters the house, and then he (the master) buys him back. They said: He does not become free.