To save for a rainy day
To put aside money with the idea of using it sometime in the future when it is needed; usually for some unexpected expense
e.g. I bought a new TV with my bonus but my father said I should save it for a rainy day.
To call hard times (when you need more money) a “rainy day” dates from the sixteenth century. Nicholas Breton used it in 1582 (Works): “Wise men say keepe somewhat till a rainy day.”