Verse 92of 110

His Journey to the Land of Ya'juj and Ma'juj, and building the BarrierAllah says of Dhul-Qarnayn:ثُمَّ أَتْبَعَ سَبَباً (Then he followed (another) way) meaning, he traveled from the east of the earth until he reached a place between the two mountains which were next to one another with a valley in between, from which Ya'juj and Ma'juj (God and Magog) will emerge into the land of the Turks and spread mischief there, destroying crops and people.

Source: Tafsir Ibn Kathir — abridged excerpt