Every evil in the bud is easily crushed: as it grows older, it becomes stronger.

[Omne malum nascens facile opprimitur; inveteratum fit pleurumque robustius.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
Philippicae [Philippics], No. 5, ch. 11 / sec. 28 (5.28) (1 Jan AD 43)
    (Source)

(Source (Latin)). Alternate translations:

Every evil is easily crushed at its birth; when it has become of long standing, it usually gets stronger.
[tr. Yonge (1903)]

Every evil is easily crushed at its birth; become inveterate it as a rule gathers strength.
[tr. Ker (1926)]