﴿ حَتَّىٰٓ إِذَا بَلَغَ بَيْنَ ٱلسَّدَّيْنِ وَجَدَ مِن دُونِهِمَا قَوْمًۭا لَّا يَكَادُونَ يَفْقَهُونَ قَوْلًۭا ﴾

“[And he marched on] till, when he reached [a place] between the two mountain-barriers, he found beneath them a people who could scarcely understand a word [of his language].”

Please check ayah 18:96 for complete tafsir.