Surah Al-Kahf 18:97

فَمَا ٱسْطَـٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَـٰعُوا۟ لَهُۥ نَقْبًا

So they [i.e., Gog and Magog] were unable to pass over it, nor were they able [to effect] in it any penetration.

Translation: Saheeh International

Tafsir

And so they Gog and Magog were not able to scale it to climb up its length because of its great height and smoothness nor could they pierce it because of its firmness and thickness.

Excerpt from Tafsir al-Jalalayn, commentary on 18:97, via Quran.com. The reader has the full commentary and 9 more tafsirs.