Sia wrote the lyrics and melody for this track in just 14 minutes.
This is likely the song you are thinking of, as it is the most famous collaboration between the two. Sia Rihanna Unstoppable
Rihanna famously mimicked Sia's vocal demo almost exactly, which Sia later said she found incredibly flattering. You can read more about the production on the Diamonds Wikipedia page . Sia wrote the lyrics and melody for this