"We were having a laugh but 48 hours later it was out and people were loving it!" Tap above to watch video report by Divya Kohli
Rapper and West Ham fan Guvna B said he was stunned after a rap he made up on the spot rocketed to the top of the streaming chart.
Guvna B, from Newham in east London, gave an impromptu performance of 'Massive' while working as a pundit for Sky Sports News.
The die-hard Hammers fan started freestyling about the club's season and the potential comings and going in the transfer market.
"As a plea to Jesse Lingard to come and sign and as a reflection on our season so far I just freestyle about how well we’re doing and I didn’t really think anything to come of it," Guvna B told ITV News London.
"I opened Twitter after the broadcast and my phone was going crazy and so many people were asking 'where can I get this freestyle… it’s amazing... the West Ham fans are loving it!' so it put pressure on me to release it and put it out as a song.
"We were having a laugh but 48 hours later it was out and people were loving it," he added.
Guvna B said getting to the top of the chart was a "massive shock" after having a "very average rap career".
But he said there was a good reason for the song's sudden success.
"I think the catalyst for that was that all proceeds were going towards Isla Caton who is a seven-year-old girl fighting neuroblastoma and a West Ham fan too so she already has a connection with the club - she already has our hearts so it felt like a good thing to do," Guvna B said.
Isla has undergone chemo, radiation, two stem cell transplants, brain surgery and an operation to remove the main tumour.
But last year Isla's family revealed the devastating news that the treatment she was receiving was no longer working.
"If you look at the last couple of years it’s been difficult for lots of people," Guvna B said.
"The question I’ve been using myself is how can I possibly help in any way and I think when you’re connected to someone like Isla I think this is my way of making the world a better place you’ve got to to for it.
"I saw that music - particularly rap music - gets a bad rep a lot of the time so why not use it in a positive way.
"It’s great football can have a bigger impact. I’ve had rival fans buying the song and promoting it because they see what the cause is.
"There aren’t many things in life that can bring people together from different backgrounds and get so many people to gather around this cause but football is one of them," he added.
Guvna B, real name Isaac Borquaye, said that all proceeds from the song would go to Isla Caton and her family.
For more details of how you can support Isla, click here.