Surah Az-Zumar 39:56

أَن تَقُولَ نَفْسٌ يَـٰحَسْرَتَىٰ عَلَىٰ مَا فَرَّطتُ فِى جَنۢبِ ٱللَّهِ وَإِن كُنتُ لَمِنَ ٱلسَّـٰخِرِينَ

Lest a soul should say, "Oh, [how great is] my regret over what I neglected in regard to Allāh and that I was among the mockers."

Translation: Saheeh International

Tafsir

so apply yourselves to this before any soul should say ‘Alas for me yā hasratā is actually yā hasratī meaning ‘O regret of mine’ for what I have neglected of my duty to God in other words of obedience to Him. …

Excerpt from Tafsir al-Jalalayn, commentary on 39:56, via Quran.com. The reader has the full commentary and 9 more tafsirs.