One must be on one’s guard against the bad man, lest he seize his opportunity.

[Τον φαύλον παραφυλάττειν δει, μη καιρού λάβηται.]

Democritus (c. 460 BC – c. 370 BC) Greek philosopher
Fragment 87 (Diels) [tr. Freeman (1948)]
    (Source)

(Source (Greek); Diels 87. (152 N.)). Other translations:

It is necessary to guard against the fool lest he find an opportunity.
[tr. @sentantiq (2018); Fr. 88]

We should guard against bad men lest they seize the opportunity.
[E.g.]

Also cited as Frag. 409 [F278] D 53 (B87).