The bystander was heard in video footage of the incident at Leytonstone Tube station in east London last weekend, shouting as an attacker wielding a knife stabbed a man.
The hashtag YouAintNoMuslimBruv went viral on Twitter and David Cameron even mentioned the phrase during a speech on Monday.
The Prime Minister said: “‘You ain’t no Muslim, bruv’ said it all much better than I could have done.”
The man behind the phrase – a 39-year-old father named only as John – said he had to voice his feelings when he saw what was happening in the station.
He said: “I saw the guy. I was like, well you ain’t a Muslim… That’s my views, and I had to let him know that, because he looked to be a terrorist. I don’t believe in all that.”
John, who is not a Muslim himself, added: “People look at Muslims, and look at ISIS, and think they’re all the same. But obviously they’re not.”
John – security guard from north London – said he was happy to hear his 20-year-old son commend him for doing “the right thing”.