Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: "O you who believe, do not follow the footsteps of Satan" [Surah al-Nur: 21]: what is meant is the slips of Satan. "For indeed he commands indecency and evil" [Surah al-Nur: 21]: by "indecency" is meant disobedience to Allah, and by "evil" everything that Allah dislikes. "And had it not been for the favour of Allah upon you and His mercy, not one of you would ever have been purified" [Surah al-Nur: 21]: that is, I would never have accepted the repentance of any of you. "But Allah purifies whom He wills" [Surah al-Nur: 21]: that is, I willed to accept your repentance. "And Allah is All-Hearing, All-Knowing" [Surah al-Nur: 21]: Hearing your words, Knowing the remorse and repentance in your hearts.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19522