See the git-pull man page:
git pull [options] [<repository> [<refspec>...]]
and in the examples section:
Merge into the current branch the remote branch next:
$ git pull origin next
So I imagine you want to do something like:
git pull origin dev
To set it up so that it does this by default while you're on the dev branch:
git branch --set-upstream-to dev origin/dev
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