It ain’t over till the fat lady sings

It ain’t over till the fat lady sings is a colloquialism which is often used as a proverb. It means that one should not presume to know the outcome of an event which is still in progress. More specifically, the phrase is used when a situation is nearing its conclusion.

https://en.wikipedia.org/wiki/It_ain’t_over_till_the_fat_lady_sings