Surah Al-Munafiqun 63:3

ذَٰلِكَ بِأَنَّهُمْ ءَامَنُوا۟ ثُمَّ كَفَرُوا۟ فَطُبِعَ عَلَىٰ قُلُوبِهِمْ فَهُمْ لَا يَفْقَهُونَ

That is because they believed, and then they disbelieved; so their hearts were sealed over, and they do not understand.

Translation: Saheeh International

Tafsir

That namely their evil deed is because they believed by affirming faith only with the tongue then disbelieved in their hearts that is to say they persist in harbouring disbelief in it; therefore their hearts have been stamped sealed with disbelief. Hence they do not understand faith.

Excerpt from Tafsir al-Jalalayn, commentary on 63:3, via Quran.com. The reader has the full commentary and 9 more tafsirs.