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 is likely the song you are thinking of, as it is the most famous collaboration between the two. Sia Rihanna Unstoppable
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. There seems to be a bit of a