I don't think that's a Pratchett original. It was a Ruthless Rap Assassins track before Reaper Man, and I suspect it wasn't original then either. If anybody knows where it originates I would like to know.
That quote was used in the Discworld novel Mort in 1986, when death was talking to his apprentice. According to Wikipedia, Ruthless Rap Assassins' first album was in 1990 and Reaper Man came out in 1991. I'm sure you are right that other's likely have said it previously.
I don't think that's a Pratchett original. It was a Ruthless Rap Assassins track before Reaper Man, and I suspect it wasn't original then either. If anybody knows where it originates I would like to know.