UI Button function

I am trying to use mouse_hovered_fn of UI button. I am not able to figure out its exact usage. I am sharing a code snippet here.

self._test_window = ui.Window("Test Window", width=300, height=200)
        with self._test_window.frame:
            with ui.VStack(height=0, spacing=4):
                self.hoverButton = ui.Button("Hover Over!", height = 54, mouse_hovered_fn = self.stop())

I am expecting self.stop() function will be executed whenever the mouse is hovered over on hoverButton button.