The rumor has been propagated on several pro-wrestling news sites, including the mostly reputable Mexican wrestling website SuperLuchas.
SuperLuchas—as well as multiple sources throughout the Internet—claims that the news was first broken by the Wrestling Observer Newsletter, despite the lack of evidence of the reputable newsletter ever publishing such news.
According to the websites which cite the alleged Wrestling Observer Newsletter report:
Lesnar left the building after getting into a physical altercation with fellow WWE superstar, Ryan Reeves, known in ring as Ryback.
Not much information is being let out, but the source says Lesnar was in the arena early to rehearse a major angle with SummerSlam opponent Triple H. Before things even began, Reeves, who was also in the building, got into a heated argument with Lesnar. It is uncertain what the argument was about.
Reeves put his hands on Lesnar, though the source tells us that people who witnessed it said Reeves was going to apologize. Lesnar shoved Reeves back and a shoving match ensued. Lesnar threw Reeves to the floor and hit him in the face three times before being pulled off.
Reeves suffered a laceration above his left eyebrow which is required stitches. 'Ryback' will not be competing tonight as he was originally planned. Lesnar left the building immediately, and all efforts to reach him have failed. 
wasnt this outed as bullshit?
its wwe so its bullshit
UGCTT_song2 - wasnt this outed as bullshit?
Yea it was.
There was another thread about this already and the general consensus due to no evidence or witnesses is it's a BS rumor
Dude can cut it in the UFC so he decided to beat up pro wrestlers.
Who does that???
Not true at all they both made appearances on Monday
Not true at all they both made appearances on Monday