By Christopher Rosen
December 25, 2016 at 03:12 PM EST
S Granitz/WireImage

Sir Elton John is one of the many celebrities paying tribute to George Michael, who died Sunday at 53.

“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.

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The two pop legends 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.

Watch the video below.

“It is with great sadness that we can confirm our beloved son, brother and friend George passed away peacefully at home over the Christmas period. The family would ask that their privacy be respected at this difficult and emotional time. There will be no further comment at this stage,” his representative said in a statement to EW.