George Foreman -- Bernard Hopkins Should Retire ... You Gotta Know When to Stop

The legendary George Foreman tells TMZ Sports he has a message for Bernard Hopkins -- HANG 'EM UP ... while you still have your health.

Foreman -- who famously fought until he was 48-years-old -- says he was incredibly impressed with 49-year-old B-Hop's commanding victory over Beibut Shumenov this weekend ... but cautions, "Don't tempt fate."

"I fought all the way until I was about 50," Foreman says ... "Around my 56th birthday, I was gonna come back. I saw a path to the heavyweight championship. My wife said, 'you're not gonna do it.' We all need someone like that. It was the best thing that could've happened to me."

Foreman adds, "Leave while you can. When the curtain closes, you don't want to be laying on your back in the middle of the ring."

When asked what he would say to Hopkins, Foreman told us, "Bernard, you looked great! Fantastic! Isn't that the way you wanna go out? You looked unbeatable, now leave."

"I think Muhammad Ali would say the same thing. Get out and smell the roses."

"There's life after boxing," George says ... joking, "If he makes a Bernard Hopkins Grill, I'll have all my kids buy one! I have 10 kids, 10 grandkids, and 2 great grandchildren. He can make a lot of money just off us!"