There seems to be a bit of a mix-up between these two artists. Sia's hit song was not recorded by Rihanna, nor did Rihanna write it. However, the two have a very famous connection through the song "Diamonds." "Unstoppable" by Sia
This song was released on Sia’s 2016 album This Is Acting . The album's concept was based on songs Sia wrote for other artists (like Adele or Rihanna) that they ultimately passed on. Sia Furler, Christopher Braide. Producer: Jesse Shatkin. Release Date: January 21, 2016. Sia Rihanna Unstoppable
This is likely the song you are thinking of, as it is the most famous collaboration between the two. There seems to be a bit of a