For reference: it's called Kernighan's Law, and can be found in the Second Edition of "The Elements of Programming Style", page 10 [1].

The original phrasing is:

> Everyone knows that debugging is twice as hard as writing a program in the first place. So if you’re as clever as you can be when you write it, how will you ever debug it?

[1] https://archive.org/details/the-elements-of-programming-styl...