Bruce Jenner to Michael Phelps -- HANG UP THE SPEEDO ... 'Quit While You're Ahead'

With rumors of a Michael Phelps comeback, there's one American who thinks MP should STAY OUT of the pool when the 2016 Olympics roll around -- Bruce Jenner.

Phelps announced he "retired" from swimming after dominating at the 2012 games ... but lately, there have been rumblings that he's considering re-shaving his body hair for a dramatic Olympic comeback.

But not everyone's fired up about the idea ... especially Jenner, who knows a little something about what it takes to compete at the Olympic level -- 'cause before he played second fiddle to the Kardashians, he won gold in the decathlon in 1976.

"He's done," Jenner told us in Calabasas yesterday ... "Quit while you're ahead."

For the record, Phelps is only 28 ... and still in incredible shape.