הלכה על עירובין 10:9

We hold that a safek isur Torah is asur, and the opinion of the Rambam in his great work [Mishneh Torah] in several places, is that this rule is only mid'rabanan, and that mid'oraisa all case of doubt are mutar. This is also the opinion of the Raavad. But the Ramban and the Rashba argue and they prove that when Chazal say "you must be stringent on a doubt of Torah prohibiton", that is mid'oraisa. And the Pri Chadash in Yoreh Deah elaborates on this. See there, section 110.