Sounds like the Kalama Sutta.
Yes, that's it.
I found it in an old book in a Buddhist library on a meditation retreat I was at. The chapter was called Buddhism and Science. The whole series called The Wheel Publication has many interesting essays but the quote from the Kesamutti Sutta aka Kalama Sutta was just too good to not share here.
This is a fun question/quiz. Who said the following?
You are in doubt in a situation in which you should be uncertain. Do not accept anything because it is rumoured to be, because it is the traditional belief, because the majority hold to it, because it is found in textbooks, because it is the product of metaphysical argument and speculation, or after a superficial investigation of facts, or because it conforms with your inclinations, because it is authoritative or because of the prestige value of your teacher.
If anyone were to speak ill of me, my codex, or my followers do not bear any ill towards them, be upset or agitated, for if you were to be so it will only cause you harm. If on the other hand anyone were to speak well of me, my codex, or my followers, do not be overjoyed, thrilled or elated, for if so it will only be in your way of forming a correct judgement as to whether the qualities praised are real and actually found.
The original quote is somewhat altered to make it harder to google and more interesting to guess.