The Quran

وَتَحسَبُهُم أَيقاظًا وَهُم رُقودٌ ۚ وَنُقَلِّبُهُم ذاتَ اليَمينِ وَذاتَ الشِّمالِ ۖ وَكَلبُهُم باسِطٌ ذِراعَيهِ بِالوَصيدِ ۚ لَوِ اطَّلَعتَ عَلَيهِم لَوَلَّيتَ مِنهُم فِرارًا وَلَمُلِئتَ مِنهُم رُعبًا

Muhammad Asad

And thou wouldst have thought that they were awake, whereas they lay asleep. And We caused them to turn over repeatedly, now to the right, now to the left; and their dog [lay] on the threshold, its forepaws outstretched. Hadst thou come upon them [unprepared], thou wouldst surely have turned away from them in flight, and wouldst surely have been filled with awe of them.¹

I.e., an accidental onlooker would immediately have felt the mystic, awe-inspiring aura that surrounded the Men of the Cave, and would have become conscious that he stood before God's elect (Tabari, Razi, Ibn Kathir, Baydawi).