Musannaf Ibn Abi Shayba 9:149

Hafs ibn Ghiyath narrated to us, from Ibn Jurayj, from ‘Ata’, regarding a woman who is divorced three times, then marries a slave without the permission of his masters. ‘Ata’ said: Every marriage that is not upon the manner of a [valid] marriage, she does not return to her first husband [through it].