I read this quote from Harry Potter in the book or on the Internet, I don't remember. I think I read it 3-4 years ago. It was like
Things are gonna get serious for you.
This quote was a play on his name.
This conversation took place between Sirius Black and someone (who was a Muggle, not a wizard).
I think he was riding his motorcycle as well.
It's a long task to go through all the novels, but I tried looking in those where Sirius appeared. However, I still couldn't find that quote.