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 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 Sia Rihanna Unstoppable

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 This is likely the song you are thinking