ABC/Randy HolmesSir Elton John is honoring Princess Diana‘s charitable spirit in the upcoming documentary, Diana, Our Mother: Her Life and Legacy.

In a new sneak peek from the film on People.com, the pop-rock legend joins Prince Harry to discuss his late mother’s AIDS and HIV activism — a cause that’s particularly close to Elton’s heart. He founded his Elton John AIDS Foundation in 1992.

“It was considered to be a gay disease,” Elton says in the clip. “For someone who was within the Royal Family and who was a woman, and who was straight, to have someone care from the other side, was an incredible gift.”

Elton goes on to praise Prince Harry for taking up his mother’s charitable causes.

“She had that incredible ability — which [Harry] kind of inherited — to make people feel at ease and make them feel that everything’s gonna be all right,” Elton says. “I haven’t experienced many people in my life who have that ability, but she could walk into a room of people and make them feel as if everything was great.”

Elton was friends with Princess Diana and re-wrote his song “Candle in the Wind” as a tribute to her following her death in 1997.

Diana, Our Mother: Her Life and Legacy airs Monday, August 31, at 10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.

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