The man allegedly punched by Conor McGregor in a Dublin boozer told the fighter: "I don’t want to drink that s***."
A video surfaced on Thursday which appeared to show the Irishman hit an elderly man sat at the bar.
McGregor had earlier bought a bottle of his own Proper Twelve whiskey before pouring three shots.
He and another man downed the drink but a third man declined the offer.
McGregor took offence and appeared to punch him in the head before he was ushered out of the pub by his minders.
A source told the Irish Mirror: "The fella McGregor offered a drink to said, ‘I don't want to drink that s***, I don't want your drink' and McGregor wasn't happy. And sure that was the end of it – then he swung for him.
“As far as I know he hasn't been in touch with the pub since it all happened. It happened back in April.
"It was stupid and maybe the man should have just passed the drink on to someone else and just thanked him."
McGregor will reportedly be charged over the alleged assault.
A source told the Irish Sun: "The file was completed, sent to the DPP and after its examination the decision has been made to charge McGregor with the lowest form of assault.
"He will now have to appear in court to answer the charge."
Earlier this year, McGregor was accused of smashing and stealing Ahmed Abdirzak's phone outside a Miami Beach hotel.
He was arrested on March 11 with police stating alleged victim Abdirzak was taking a picture of McGregor outside a hotel when he slapped the man's phone out of his hand.
But in May Abdirzak dropped the lawsuit and reached an out-of-court settlement with McGregor.
McGregor has not fought in the octagon since he was submitted by lightweight champion Khabib Nurmagomedov last October.
He could return early next year against Nate Diaz if he wins his fight against Anthony Pettis on Saturday.
