Canelo Alvarez hit back at Ali Abdelaziz - Usman's manager - after he claimed the four-division world champion was scared of a crossover fight against him.
The Mexican - who is bidding to become light heavyweight champion against Dmitry Bivol on Cinco de Mayo weekend - replied to his tweet and said: "Who the f**k is this?"
Usman then chipped in and added: "Let's just all relax… before somebody gets hurt."
Canelo continued the discussion by quoting Usman and said: "I agree cuz it will not be me."
The beef continued with Usman - UFC's first African champion - asking if Canelo was open to fighting him in the cage or boxing ring only for the 31-year-old to go into complete savage mode.
Alvarez asked: "You calling me out, right? You want the payday, right?
"So you know where, but sit down it's not your turn yet I have a legacy to make."
Usman got the last word in and concluded: "Enough said… you don't want that smoke in a real fight. I hear you! Good luck champ."
Usman is short of challenges in MMA following his win over Colby Convington at UFC 268 and is now prepared to train for a showdown against Canelo.
However, he reassured fans that his hopes of securing the fight is not for financial gain.
He recently told SunSport: "The more it scares me, the more I'm willing to prepare and go through [hell] to achieve it.
"It's no surprise. Canelo is a tall glass for anybody - a tall pint.
"And it scares me, it scares me to death. But the real reason [I want the fight] is: I wanted to be welterweight champion, I got it.
"I wanted to be pound-for-pound, I got it. I always thought this phrase was special, and the phrase that I'm after now is: 'The Face of the Fight Game'.
"And Canelo, right now, can't say [he's the face of the fight game].
"Yeah, he's the face of boxing, but he can't say he's the face of the fight game.
"And I can't claim to that as well. So, there's only one way for me to stake claim to that.
"And that's taking on something that seems astronomically impossible to everyone, except myself."