Appears 2 times in the Quran.
And they say: Our hearts are covered. Nay, Allah has cursed them on account of their unbelief; so little it is that they believe.
4:155 فَبِمَا نَقْضِهِم مِّيثَٰقَهُمْ وَكُفْرِهِم بِـَٔايَٰتِ ٱللَّهِ وَقَتْلِهِمُ ٱلْأَنۢبِيَآءَ بِغَيْرِ حَقٍّ وَقَوْلِهِمْ قُلُوبُنَا غُلْفٌۢ بَلْ طَبَعَ ٱللَّهُ عَلَيْهَا بِكُفْرِهِمْ فَلَا يُؤْمِنُونَ إِلَّا قَلِيلًا
Therefore, for their breaking their covenant and their disbelief in the communications of Allah and their killing the prophets wrongfully and their saying: Our hearts are covered; nay! Allah set a seal upon them owing to their unbelief, so they shall not believe except a few.