The Flash, Season 3

Jay:
What you saw was one possibility of the future. What may be, or what may not.

Barry:
Okay, then I can stop this from happening. I can run back there --

Jay:
You can't go back there! Not again. Not ever.

Barry:
All right, then I'll figure out exactly how it happened, how we wind up on that street, and I'll just make sure that we don't go there. I mean, if we don't go there...

Jay:
Barry! This is why speedsters don't travel to the future. Nobody should know this much about their own. Just as there are infinite Earths in the multiverse, there are infinite possibilities to the future. It's always bending, always changing. Every decision you make creates another alternative.

Barry:
I won't let Iris die.

Jay:
The future isn't written yet. And it might not even turn out to be what you saw. You need to focus on the here and now. You have to live your life.