Elton John joined the celebrities mourning George Michael’s death.

53-year-old Michael died on Sunday, most likely due to heart failure. His family asked for privacy after this tragic event.

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“I am in deep shock. I have lost a beloved friend — the kindest, most generous soul and a brilliant artist. My heart goes out to his family and all of his fans,” John wrote on Instagram alongside a photo of him and Michael.

The two singers were friends for years and collaborated together on a cover of John’s hit “Don’t Let the Sun Go Down on Me” in 1991. The song hit the top of the U.S. and UK charts.