Δειλαί τοι δειλῶν γε καὶ ἐγγύαι ἐγγυάασθαι.
A bad man’s bond is bad security.
Homer, The Odyssey, Book VIII (Samuel Butler, trans., The Odyssey of Homer, Rendered Into English Prose, 1900), Book VIII. This is variously translated; see a past quotation here.