Gaius, i, 52 ff. Cf. Ulpian in Dig., 1, 12, 1 and 8.
The punishment for this was pecuniary damages equal to twice the highest value of a slave during the year in which he was killed.
Ulpian in Dig., i., 12, 8: hoc quoque officium praefecto urbi a divo Severo datum est, ut mancipia tueatur ne prostituantur.