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Sharing a photo of Nurmagomedov and one of his wife, McGregor tweeted: "Your wife is a towel mate."
McGregor deleted the tweet within around 15 minutes, but not before it racked up tens of thousands of reactions - and angry comments from many fellow social media users, who said the message was 'racist' and 'Islamophobic'.
Sharing a screenshot of the deleted tweet, one person wrote: "Remember when people tried to argue that Conor McGregor isn't racist and the things he said were just 'banter'."
Another person tweeted: "You're a racist scumbag for calling Khabib's wife a towel. You may have deleted the tweet but it's too late. I pray for your own good you never cross paths with him or it's game over for you, this time possibly for good 'mate'. Actions have consequences."
Someone else said: "#ConorMcGregor is a massive racist. Though we knew he was already, he has just stepped it up a gear. We know full well what happens when that kind of language is normalised."
A fourth added: "Nothing worse than seeing someone you idolised show their true colours a racist c**t. @TheNotoriousMMA stay retired."
And another Twitter user said: "Conor McGregor is a man with no dignity or honour... that's it... just look at the racist Islamophobic comments."
In another tweet, McGregor also referred to Nurmagomedov as a 'little scurrying rat', telling him not to be scared of the rematch.
Posting a message on Twitter, he wrote: "Hey guys quick announcement, I've decided to retire from the sport formally known as 'Mixed Martial Art' today. I wish all my old colleagues well going forward in competition.
"I now join my former partners on this venture, already in retirement. Proper Pina Coladas on me fellas!"
Initially, fans weren't particularly convinced when the news broke, with many believing that McGregor will end up like his one-time opponent Floyd Mayweather and simply come out of retirement when the offers get too good to refuse.
Plus, it's also not the first time Conor has teased his retirement. In 2016, the UFC fighter wrote on Twitter: "I have decided to retire young. Thanks for the cheese. Catch ya's later."
Nurmagomedov also wasn't convinced, saying McGregor was like a 'jealous wife' who always returns.
Speaking on Tuesday at a Moscow university, Nurmagomedov said McGregor 'couldn't reach a deal with the UFC, and [his retirement] is so they feel they've lost him and they could lose money.'
"I don't think he's finished," Nurmagomedov said. He added: "Conor acts like a jealous wife who says 'I will leave' all the time but then comes back."
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