Tafseer Ibn Kathir of AL KAHFI [12]

ثُمَّ بَعَثْنَاهُمْ لِنَعْلَمَ أَيُّ الْحِزْبَيْنِ أَحْصَى لِمَا لَبِثُوا أَمَدًا

Then We awakened them that We might show which of the two factions was most precise in calculating what [extent] they had remained in time.