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
Sa'id bin Jubayr said: "O you who believe, do not follow the footsteps of Satan" [Surah al-Nur: 21]: that is, the embellishment of Satan, until he got Sayyidah Aishah slandered. "And whoever follows the footsteps of Satan" [Surah al-Nur: 21]: that is, the embellishment of Satan, "then indeed he commands indecency" [Surah al-Nur: 21]: that is, acts of disobedience, "and evil": that is, something that is not recognised (as right), meaning the like of what was said about Sayyidah Aishah. "And had it not been for the favour of Allah upon you and His mercy" [Surah al-Nur: 21]: that is, His blessing, "not one of you would ever have been purified" [Surah al-Nur: 21]: that is, not one of you would ever have been made upright. "But Allah purifies whom He wills" [Surah al-Nur: 21]: that is, He makes upright whom He wills.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19523