Tafseer Ibn Kathir of AZ ZUMAR [56]

أَن تَقُولَ نَفْسٌ يَا حَسْرَتَى علَى مَا فَرَّطتُ فِي جَنبِ اللَّهِ وَإِن كُنتُ لَمِنَ السَّاخِرِينَ

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