防患于未然
Prevention is better than cure.
Nip in the bud.
Crush in the egg.
To take precaution.

eg：Thieves is common in this area. Better lock the door as prevention is better than cure.

中英俚语俗语来源：汉书·外戚传：“事不当固争，防祸于未然。”

同义词：防微杜渐 A little fire is quickly trodden out.
反义词：亡羊补牢 Lock the stable door after the horse is stolen.