(Al-Kahf) 18 : 98
قَالَ هَٰذَا رَحْمَةٌ مِنْ رَبِّي ۖ فَإِذَا جَاءَ وَعْدُ رَبِّي جَعَلَهُ دَكَّاءَ ۖ وَكَانَ وَعْدُ رَبِّي حَقًّا
He said: "This is a Mercy from my Lord; but when the Promise of my Lord comes, He will make it level, and the Promise of my Lord is ever True".
Being a believer, Dul-Qarnayn submits everything to Lord just Angel Khidr did. The Promise of the Lord in this verse means the Doomsday.