﴿ جَزَآءًۭ مِّن رَّبِّكَ عَطَآءً حِسَابًۭا ﴾

“[All this will be] a reward from thy Sustainer, a gift in accordance with [His Own] reckoning –”

this will be a reward from your Lord in other words God has rewarded them with this reward as a gift ‘atā’an substitutes for jazā’an ‘reward’ that is sufficing that is abundant hisāban as meaning ‘abundant’ derives from their saying a‘tānī fa-ahsabanī ‘he gave me so abundantly that I said “that is enough for me”’ hasbī