Surah Al-Kahf, Ayah 104
Quran 18:104 · The Cave · Makki · Para 16 · Page 304
اَلَّذِیْنَ ضَلَّ سَعْیُهُمْ فِی الْحَیٰوةِ الدُّنْیَا وَهُمْ یَحْسَبُوْنَ اَنَّهُمْ یُحْسِنُوْنَ صُنْعًا ۟
Transliteration
Allatheena dalla saAAyuhumfee alhayati addunya wahum yahsaboonaannahum yuhsinoona sunAAa
Saheeh International
[They are] those whose effort is lost in worldly life, while they think that they are doing well in work."
Al-Mukhtasar fi Tafsir
Those who will see on the Day of Judgment that the effort they used to make in the world has gone to waste, whilst they thought they were doing well in their effort and that they will benefit from their actions. The reality was the exact opposite.
Tafsir Ibn Kathir (abridged)
Explains 18:103–18:106 together — read it on 18:103