Plato, Laws 882a

τῶν κατηγορημάτων τῶν μεγίστων ἓν τοῦτο αὐτῷ. ἐὰν δὲ αὖ δοῦλος τύπτῃ τὸν ἐλεύθερον, εἴτʼ οὖν ξένον εἴτε ἀστόν, βοηθείτω μὲν ὁ προστυγχάνων ἢ κατὰ τὸ τίμημα τὴν εἰρημένην ζημίαν ἀποτινέτω, συνδήσαντες δὲ οἱ προστυγχάνοντες μετὰ τοῦ πληγέντος παραδόντων τῷ ἀδικουμένῳ· ὁ

If it be a slave that strikes the free man—stranger or citizen—the bystander shall help, failing which he shall pay the penalty as fixed according to his assessment; and the bystanders together with the person assaulted