Qur'ān 39:56 · Az-Zumar (الزمر)

أَن تَقُولَ نَفْسٌۭ 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 Allah and that I was among the mockers."