Laravel update parent of parent updated_at timestamp

Question

I am trying to update a parent of a parents updated_at timestamp when the child model is updated. I can updated the parent model using the following code inside the child model

protected $touches = ['post']

but for example if I also wanted to update the user who made the posts updated_at timestamp how would I do this?

I was hoping I could just add the following code to the Post.php model

protected $touches = ['user']

but this doesn't seem to work, the user model timestamp only gets updated if the Post record is updated.