Surah Al-Kahf 18:93

حَتَّىٰٓ إِذَا بَلَغَ بَيْنَ ٱلسَّدَّيْنِ وَجَدَ مِن دُونِهِمَا قَوْمًا لَّا يَكَادُونَ يَفْقَهُونَ قَوْلًا

Until, when he reached [a pass] between two mountains, he found beside them a people who could hardly understand [his] speech.

Translation: Saheeh International

Tafsir

until when he reached between the two barriers read al-saddayn or al-suddayn here and likewise further below at verse 94 saddan or suddan — these were two mountains lying in the remote regions beyond the land of the Turks; …

Excerpt from Tafsir al-Jalalayn, commentary on 18:93, via Quran.com. The reader has the full commentary and 9 more tafsirs.