Musannaf Ibn Abi Shayba 19:105

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Abu Hurrah, from al-Hasan, who said: The confession of a slave is permissible regarding a prescribed punishment (hadd) that is to be carried out upon him. And whatever he confesses to, by which his neck (i.e., his life or his status as property) would be lost, it is not permissible.