discrete mathematics – How to prove a proposition related to sets operation

I am asked to prove or disprove the following proposition:
$$ X cap Y = X implies X cup Y = Y$$

I feel this is true, because this means that X is a subset of Y, and as a result, when we do union with these two, it should be the entire set.

However, I have a hard time constructing a formal proof to prove this. Not sure where to start from.