Surah Al-Kahf — Ayah 104 (18:104)
ٱلَّذِينَ ضَلَّ سَعۡيُهُمۡ فِي ٱلۡحَيَوٰةِ ٱلدُّنۡيَا وَهُمۡ يَحۡسَبُونَ أَنَّهُمۡ يُحۡسِنُونَ صُنۡعًا ١٠٤
English translation (Saheeh International)
[They are] those whose effort is lost in worldly life, while they think that they are doing well in work."
Brief commentary
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.