Lord, you're not going to make me look that up, are you?
Seriously, it's been awhile... but I can say with certainty that I've read it in several sources, and heard it on some podcasts, too. But, being trained in evidence-based medicine, I know that's technically not good enough!
Personally, I buy a 1-gallon jug of distilled water, add 6 mL of Starsan to it, shake it up, and keep it under the sink. I'd say I use, on average, about 1 L per every session of brewing and bottling, and fill up the squirt bottle as needed.
If I can find a back-up source, I'll post 'er.