Surah Al-Fath 48:12

بَلْ ظَنَنتُمْ أَن لَّن يَنقَلِبَ ٱلرَّسُولُ وَٱلْمُؤْمِنُونَ إِلَىٰٓ أَهْلِيهِمْ أَبَدًا وَزُيِّنَ ذَٰلِكَ فِى قُلُوبِكُمْ وَظَنَنتُمْ ظَنَّ ٱلسَّوْءِ وَكُنتُمْ قَوْمًۢا بُورًا

But you thought that the Messenger and the believers would never return to their families, ever, and that was made pleasing in your hearts. And you assumed an assumption of evil and became a people ruined."

Translation: Saheeh International

Tafsir

Nay bal in both instances marks a transition from one subject to another but you thought that the Messenger and the believers would never return to their families and that thought was adorned in your hearts that is to say you thought that they would be annihilated by being slain and would not return and you thought …

Excerpt from Tafsir al-Jalalayn, commentary on 48:12, via Quran.com. The reader has the full commentary and 9 more tafsirs.