I don't know where it came from, but I've always preferred this variant:

There are only two hard things in Computer Science: cache invalidation, naming things, and off-by-1 errors.

I actually used that version in my recent book. You can see it in this section [0] of the first chapter (which is free to read, with no sign-up) that tries to explain the confusing naming.

[0] https://www.packtpub.com/packtlib/book/Application%20Develop...

Source: https://twitter.com/secretGeek/status/7269997868 ;)

That tweet dates from 2010 and the same quote is on this page [0] dated 2009. Of course, it's possible Martin updated it after the initial publication. I'm not saying you copied it either, it could have been independently created.

[0] http://martinfowler.com/bliki/TwoHardThings.html