August 19, 2014 9:27 AM EDT

David Letterman returned to the Late Show on Monday and paid tribute to his friend, the late Robin Williams, who had died while the show was on a summer break.

Letterman had known Williams for 38 years. They met at the the Comedy Store in West Hollywood, where Letterman says he watched Williams’ first set at the club.

During his tribute, Letterman lamented Williams’ passing and his depression. “I had no idea that the man was in pain, that the man was suffering,” he said.

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