'Sippin' on Syrup' Rapper -- Justin Bieber's Gotta Learn ... SIZZURP KILLS

Three 6 Mafia rapper DJ Paul -- famous for his song "Sippin' on Some Syrup" -- has some words of advice for Justin Bieber ... stay away from sizzurp, or you could die.

Paul was out in Vegas this week when we asked him about the photos circulating of Justin Bieber -- showing the singer surrounded by double cups and codeine -- and the rapper tells us, Bieber's playing with fire.

Paul says, "I can't say don't do [sizzurp] because I did my rounds with it. But I stay away from it these days because I had a lot of friends that passed away from it. It's dangerous if you do too much of it."

He added, "But everything's dangerous if you do too much of it. Sex. Water." A less convincing point.

As for concerns Bieber's new friends Lil Za and Lil Twist are bad influences -- Paul said, "Nah, friends are friends ... It's just a phase they're going through."