That was the WARNING Muhammad Ali gave to George Foreman around 1987 ... when a 40-year-old Foreman was considering a boxing comeback after a 10-year hiatus.

"George, that Mike Tyson can hit! That's punching power there!"

Foreman tells the whole story to TMZ Sports ... explaining how Muhammad was like his "big brother" and tried to advise him to stay away from Tyson, who was DESTROYING people at the time.

"It was like a warning, 'Don't do that George!'"

Of course, Foreman DID make his comeback and went on a 23-FIGHT WIN STREAK ... crushing just about everyone until he faced Evander Holyfield in 1991.

In fact, 45-year-old George beat Michael Moorer for the WBA and IBF Heavyweight title in 1994 -- while Tyson was still in prison.

We asked George what would have happened if Tyson and Muhammad Ali ever squared off ... and ya gotta hear his answer to that too!!!

There's more ... Foreman also breaks down the upcoming Andy Ruiz vs. Anthony Joshua rematch and tells us why it would be very wise for Anthony Joshua to do some film study of Ali vs. Spinks II, the epic fight from 1978.