I probably should've just Googled it as it was easy enough to find. According to Wikipedia [1], it was Tom Cargill of Bell Labs and it's called the 90-90 rule:
> The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time
> The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time
[1] https://en.wikipedia.org/wiki/Ninety-ninety_rule