Getty

Sam Rivers -- the bassist for the rock-rap group Limp Bizkit -- has died ... according to an official statement posted to the band's Instagram.

The group announced the news Saturday evening ... revealing Sam passed away earlier in the day and calling him their "heartbeat." Instagram

Sam wasn't just a bass player, the group say -- he "was pure magic. The pulse beneath every song, the calm in the chaos, the soul in the sound."

Outside of the music, the band writes that Sam was a once-in-a-lifetime person and a legend among legends.

The message ends, "Rest easy, brother. Your music never ends."

Rivers grew up playing with Limp Bizkit drummer John Otto ... and, he met Fred Durst while working at Chick-fil-A in the early 1990s.

Originally, Rivers, Otto and Durst made up all of

See Full Page