κερδῶν ἄθικτος

From LSJ

ἐπέμψατε ἀγγέλους τοῖς ἀλλήλοις ὥστε ἔγνωτε τὸν κίνδυνον → you sent messengers to one another so that you knew the danger

Source

Greek > English (Woodhouse)

impervious to bribery, not to be bribed