With a camera rolling, a bystander appears to have shouted the phrase at the alleged attacker, a 29-year-old man, while he was being subdued by police. The footage was posted on the internet, and it instantly spiraled across Twitter, capturing the mood of London and beyond.
"#YouAintNoMuslimBruv + Met Police arresting terrorist with tasers = London Pride," one tweet read.
"The hashtag #YouAintNoMuslimBruv makes me proud to be a British Muslim," read another. "No real Muslim kills/harms innocent people,end of."
Two people were injured during the attack at Leytonstone underground station, though their injuries did not appear to be life-threatening.
The stabbing came days after British lawmakers voted to join the U.S.-led aerial campaign against the Islamic State; minutes after the vote, Royal Air Force bombers began striking oil fields in Syria that the British Ministry of Defense described as one of the terrorist groups "largest and most important" sources of income.
A witness who saw the stabbing told NBC News' U.K. partner ITN that the alleged attacker repeatedly shouted "This is for Syria" after he was detained by authorities.
By Sunday afternoon, the analytics company Topsy had clocked roughly 82,000 tweets with the "Bruv" hashtag.