Surah Hud — Ayah 65 (11:65)
فَعَقَرُوهَا فَقَالَ تَمَتَّعُواْ فِي دَارِكُمۡ ثَلَٰثَةَ أَيَّامٖۖ ذَٰلِكَ وَعۡدٌ غَيۡرُ مَكۡذُوبٖ ٦٥
English translation (Saheeh International)
But they hamstrung her, so he said, "Enjoy yourselves in your homes for three days. That is a promise not to be denied [i.e., unfailing]."
Brief commentary
They denied what he said and harmed her, pushing their denial to extremes. So Ṣāliḥ told them to enjoy life in their land for three days from the time they harmed her, then the punishment of Allah would come to them; and the coming of His punishment after that was a certain promise that there would be no possibility of denying, as it would come true.