Plato, Laws 932d

ἐὰν δέ τις ἀδυνατῇ κακούμενος φράζειν, ὁ πυθόμενος τῶν ἐλευθέρων ἐξαγγελλέτω τοῖς ἄρχουσιν ἢ κακὸς ἔστω καὶ ὑπόδικος τῷ ἐθέλοντι βλάβης. ἐὰν δὲ δοῦλος μηνύσῃ, ἐλεύθερος ἔστω, καὶ ἐὰν μὲν τῶν κακούντων ἢ κακουμένων δοῦλος, ὑπὸ τῆς ἀρχῆς ἀφείσθω, ἐὰν δέ τινος ἄλλου τῶν πολιτῶν, τὸ δημόσιον ὑπὲρ αὐτοῦ τιμὴν τῷ κεκτημένῳ καταβαλλέτω· τοῖς ἄρχουσιν δὲ ἐπιμελὲς ἔστω μή τις ἀδικῇ τὸν τοιοῦτον τιμωρούμενος τῆς μηνύσεως ἕνεκα.

If any parent when maltreated is unable to report the fact, that free man who hears of it shall inform the magistrate, failing which he shall be esteemed base, and shall be liable to an action for damage at the hands of anyone who chooses. If a slave gives information he shall be set free: he shall be set free by the Board of Magistrates if he be a slave of either the injured party or the injurers; but if he belong to any other citizen, the State Treasury shall pay his owner a price for him; and the magistrates shall take care that no one does injury to such a man in revenge for his giving information.