Detecting when back button is tapped in navigation bar

I need to resignFirstResponder() on a text field when the user taps the back button in the navigation bar of the navigation controller, otherwise I get some error. The back button works as it should (the previous view gets shown) but I don't know where to do resign first responder. It's too late if I do it in viewWillDisappear() (I tried), and prepareForSegue() doesn't get called, so I need to somehow do it as soon as the back button gets tapped or at least before viewWillDisappear(). How do I detect that event?