Surah Luqman 31:9

خَـٰلِدِينَ فِيهَا ۖ وَعْدَ ٱللَّهِ حَقًّا ۚ وَهُوَ ٱلْعَزِيزُ ٱلْحَكِيمُ

Wherein they abide eternally; [it is] the promise of Allāh [which is] truth. And He is the Exalted in Might, the Wise.

Translation: Saheeh International

Tafsir

abiding therein khālidīna fīhā is a circumstantial qualification of a future status in other words their abiding therein will have been ordained once they enter it — a promise of God in truth that is to say God promised them this and realised it in truth; …

Excerpt from Tafsir al-Jalalayn, commentary on 31:9, via Quran.com. The reader has the full commentary and 9 more tafsirs.