I found this:
"So, [found and excerpted from website eretzhemdah when googling for the famous quote]
consider :
Chazal concluded: “Just as it is forbidden to rule that the impure is pure, so is it forbidden to rule that the pure is impure” (Yerushalmi, Terumot 5:3). Poskim apply this idea generally to forbidding the permitted, (see Beit Yosef, YD 115; Aruch Hashulchan, YD 242:66). It is better to have 10 tefachim that stand than 100 that fall. Therefore, in creating walls of halacha, we mustn’t build so high that it threatens the structure. Rav Kook learns from this as follows. “One should not look to be stringent without a tradition from our teachers in a matter that is itself a distancing from sin. A stringency should not be built onto a stringency unless we have found it explicitly or we have clear proofs” (Orach Mishpat, OC 112).
Kabbalah teachings also indicate that one should be careful not to be stringent without a source. Rav Kook quotes Rav Chaim Vital as saying: “About such people it says: ‘They are wise to do bad [forbid] and they do not know how to do good [permit].’ Because they scorn the tree of life, Hashem does not help them, and they err in the details of the tree of life and the tree of knowing good and bad and turn it into bad, and they make impure that which is pure and forbid that which is permitted.”
[end of excerpt]"