This is likely the song you are thinking of, as it is the most famous collaboration between the two.
Rihanna recorded it for her 2012 album Unapologetic . Producers: Benny Blanco and Stargate. 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. This is likely the song you are thinking
Sia wrote the lyrics and melody for this track in just 14 minutes. This song was released on Sia’s 2016 album This Is Acting
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
While many songs on this album were intended for others, Sia has not publicly confirmed if "Unstoppable" was specifically written for Rihanna. "Diamonds" by Rihanna
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 .