Based on this article I found this method easier for my usecase.
My 'dev' branch was ahead of 'origin/dev' by 96 commits (so these commits were not pushed to the remote yet).
I wanted to squash these commits into one before pushing the change. I prefere to reset the branch to the state of 'origin/dev' (this will leave all changes from the 96 commits unstaged) and then commit the changes at once:
git reset origin/dev
git add --all
git commit -m 'my commit message'