لِيَجْزِي اللّهُ كُلَّ نَفْسٍ مَّا كَسَبَتْThat Allah may requite each person according to what he has earned.meaning, on the Day of Resurrection.Allah said in another Ayah,لِيَجْزِىَ الَّذِينَ أَسَاءُواْ بِمَا عَمِلُواْThat He may requite those who do evil with that which they have done. (53:31)Allah said here,إِنَّ اللّهَ سَرِيعُ الْحِسَابِTruly, Allah is swift at reckoning.when He wills to reckon a servants of His, for He knows everything and nothing ever escapes His observation. Verily, His power over all of His creation is the same as His power over one creature,مَّا خَلْقُكُمْ وَلَا بَعْثُكُمْ إِلاَّ كَنَفْسٍ وَحِدَةٍThe creation of you all and the resurrection of you all are only as a single person. (31:28)And this is why Mujahid said, سَرِيعُ الْحِسَابِ (swift at reckoning),means "keeping count.