التفسير التبليغي
(Nay! If you knew with certain knowledge!) (5)
He emphasized the rebuke with the third "Nay!" (كَلَّا), thus rebuking them three times to stress the importance of working for the religion. And He threatened them twice regarding this, to emphasize the importance of preparing for the Hereafter.
(If you knew) If you knew the matter with certainty, what you knew would surely distract you from accumulation and boasting.
(certain knowledge): Knowledge that brings solace to the heart.
Source: التفسير التبليغي