Tafsir of Al-Anbiya' 21:94

"So whoever does [some] of the righteous deeds while he is a believer - no denial will there be for his effort, and indeed We, of it, are recorders."

"Kufrān" (denial/ingratitude): This is a metaphor for the deprivation of reward, just as "shukr" (gratitude) is a metaphor for the granting of it. This is why it is said that Allah is "Shakūr" (Appreciative).

He used the negation of the genus (lā kufrāna) to be more emphatic than saying "his effort will not be denied."

"And indeed We, of it, are recorders": Meaning, We are the ones who record that effort and establish it in the scroll of his deeds. Whatever We record is not lost, and its author will be rewarded for it.

"And it is forbidden to a town which We have destroyed that they will [ever] return. Until when [the dam of] Gog and Magog has been opened and they, from every elevation, descend."