GitHub Action - trigger on push from branch A to branch B, but not fork to branch B

My Repo has Branches Apple, Banana & master.

I want to run the action when I push from Branch Apple to Branch Banana, but NOT when Pull Requests are merged into Branch B.

Current Draft of the action starts as;

on:
  push:
    branches:
      - Banana

But, if I merge a PR from a fork into branch Banana, the action will still trigger - How can I stop this?

I've seen the Workflow Syntax about using an IF statement (HERE), but that has no option for "from branch Apple"