Musannaf Ibn Abi Shayba 19:110

Abu Bakr narrated to us, he said: Mu'adh narrated to us, from Ash'ath, from al-Hasan, that he used to say: The confession of a slave against himself is not permissible if it reaches the point of [implying] a soul [i.e., capital punishment], neither in error nor in intentional [homicide].