The Cow - 2:88
"They even said: "We are fixed-minded". Yet Allah had cursed them for their disbelief! That is why few of them believe."
They said, "We are fixed-minded." (related)
The Women/4:155
"Allah sealed their understanding because of their breaking their word, because they disbelieved in the signs and proofs of Allah, because they killed the prophets unjustly, and because they said: "We are fixed-minded", yes, because of their disbelief. Therefore few of them believe."
Explained in Detail/41:5
"They said: "We are fixed in our minds against what you call us to, and there is a heaviness in our ears and a veil between us and you. Do what you will, and we will do it."