Surah An-Naba 78:5

ثُمَّ كَلَّا سَيَعْلَمُونَ

Then, no! They are going to know.

Translation: Saheeh International

Tafsir

Again no indeed! They will come to know! reiterated for emphasis; thumma is added here to declare that the second threat of chastisement is more severe than the first. God exalted be He then alludes to His power to resurrect saying

Excerpt from Tafsir al-Jalalayn, commentary on 78:5, via Quran.com. The reader has the full commentary and 9 more tafsirs.