Surah Al-Kahf 18:104

ٱلَّذِينَ ضَلَّ سَعْيُهُمْ فِى ٱلْحَيَوٰةِ ٱلدُّنْيَا وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا

[They are] those whose effort is lost in worldly life, while they think that they are doing well in work."

Translation: Saheeh International

Tafsir

Those whose effort goes astray in the life of this world those whose deeds are invalid while they reckon they think that they are doing good work good deeds for which they will be rewarded.

Excerpt from Tafsir al-Jalalayn, commentary on 18:104, via Quran.com. The reader has the full commentary and 9 more tafsirs.