Great catch. Upvoted.

I've heard it might be a rough paraphrase of a quote from the Kalama Sutta, but in its original form, it would not qualify as a "rationality quote"; it's more a defense of belief in belief, advising people to accept things as true based on whether believing it is true tends to increase one's happiness.

I actually don't think this is right though. I'm pretty sure the original form is about the importance of personal knowledge from direct experience. I think the wikipedia article makes this clear, actually. I suppose you're taking your reading from:

Kalamas, when you yourselves know: "These things are good; these things are not blamable; these things are praised by the wise; undertaken and observed, these things lead to benefit and happiness," enter on and abide in them.'

But the emphasis here should be on "when you yourselves know", not "these things lead to benefit and happiness". Keep in mind the kind of teachings being addressed are often strategies for happiness so it makes sense to be concerned with whether or not a teaching really does increase happiness.

I don't see why we can't take it as an injunction to trust only experiment and observation. It seems about right to me.

(ETA: Except of course he's talking about meditation not experiment and ignores self-deception, placebo effect, brain diversity and the all important intersubjective confirmation, but I'll take what I can get from the 5th century B.C.E.)