اسلام عليكم I heard a clip from fawzhan that if one has doubt about a single letter of the Quran he’s a kafir could you please expand on this more specifically a doubt as I overthink and wish to apply this properly.

Ustadh AbdulAziz al-Haqqan

If u suffer from waswas u should never think about this. We believe in the Qur'an to be the words of Allah and we don't doubt any single thing in it alhamdulilah.