Carl Reiner is mourning the loss of his longtime friend, legendary comedian Don Rickles.
Rickles died of kidney failure on Thursday morning, his rep confirmed to ET. He was 90.
Reiner, 95, tells ET the two had known each other since Rickles came to California to pursue his career, when he was still unknown.
"I knew Don Rickles from the early, early days of his career when he came to California," Reiner says. "I was doing a program with Ernie Kovacs, who told me to see a new comic. I went with Frank Sinatra to see this young comedian, Don Rickles. I got to know him well after that."
"He did my movie, Enter Laughing, a key scene in the movie," he continues. "I just saw the movie on TV last week and called him to tell him about it, but was not able to speak with him at that time. What people don't know is that he was a serious actor who studied acting in New York City and he was a very dear person."
Reiner says there was a side of Rickles that his fans rarely saw.
"There were two sides to Don: the professional comedian-actor, and, when you talked with him about his wife, Barbara, and his family, a very soft side," he shares. "He was a very sweet person and this is sad news."
Rickles is survived by his wife of 52 years, Barbara, as well as his daughter, Mindy Mann, and two grandchildren.
Rickles' fellow comedians and peers are clearly taking the loss hard, many of them mourning the comedy legend via social media.
"Ninety years with Don Rickles weren't enough," Jimmy Kimmel tweeted. "One of the sweetest and most lovely people I had the pleasure of knowing. We miss you already."
"A God died today. Don Rickles, we did not want to ever lose you. Never," Tom Hanks also tweeted.
Rickles' funeral services will be private, though in lieu of flowers, the family suggests that donations be made to the Larry Rickles Endowment Fund at Children's Hospital Los Angeles.
-- Reporting by Claudia Cagan