This comes in a speech by Brutus in Shakespeare’s _Julius Caesar_ :

> There is a tide in the affairs of men,
> Which, taken at the flood, leads on to fortune;
> Omitted, all the voyage of their life
> Is bound in shallows and in miseries.
> On such a full sea are we now afloat;
> And we must take the current when it serves,
> Or lose our ventures.

There is a right moment to undertake something successfully and if we fail to
grasp it, we do so at our peril. Other proverbs on this theme are listed under
Opportunity seldom knocks twice.