Both are correct. "Time will tell" is the more commonly used expression. It just sounds better, maybe because of the two t's.

Time will tell

As a native speaker (from Scotland), I rarely hear/read 'time will show', So I have the feeling 'time will tell' is a lot more common. I double checked with Ngram Viewer and it came back pretty much as I expected: https://books.google.com/ngrams/graph?content=time+will+tell%2C+time+will+show&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3

I think 'time will tell' is more correct. I'm not a native speaker, but I read it in my English books written by a native speaker

Time will tell