Tafsir of Al-Anbiya' 21:94

فمن يعمل من الصالحات وهو مومن

So whoever does righteous good deeds while he is a believer, meaning, his heart believes and his deeds are righteous.

فﻼ كفران لسعيه

his efforts will not be rejected. This is like the Ayah:

إنا ﻻ نضيع أجر من أحسن عمﻼ

certainly We shall not make the reward of anyone who does his deeds in the most perfect manner to be lost. (18:30) which means, his efforts will not be wasted; they will be appreciated and not even a speck of dust's weight of injustice will be done. Allah says:

وإنا له كاتبون

Verily, We record it for him. means, all his deeds are recorded and nothing of them at all is lost