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Surah 23. Al-Muminun
Ayah 117 - 118
117 وَمَن يَدْعُ مَعَ ٱللَّهِ إِلَـٰهًا ءَاخَرَ لَا بُرْهَـٰنَ لَهُۥ بِهِۦ فَإِنَّمَا حِسَابُهُۥ عِندَ رَبِّهِۦٓ ۚ إِنَّهُۥ لَا يُفْلِحُ ٱلْكَـٰفِرُونَ
118 وَقُل رَّبِّ ٱغْفِرْ وَٱرْحَمْ وَأَنتَ خَيْرُ ٱلرَّٰحِمِينَ
117. And whoever invokes besides Allah, any other god, of whom he has no proof; then his reckoning is only with his Lord. Surely, disbelievers will not be successful. 118. And say : "My Lord! Forgive and have mercy, for You are the Best of those who show mercy!''
(of whom he has no proof), meaning no evidence for what he says. Then Allah says:
(And whoever invokes, besides Allah, any other god, of whom he has no proof;) this is a conditional sentence, whose fulfilling clause is:
(then his reckoning is only with his Lord.) meaning, Allah will call him to account for that. Then Allah tells us:
(Surely, disbelievers will not be successful.) meaning, they will not be successful with Him on the Day of Resurrection; they will not prosper or be saved.
(And say: "My Lord! Forgive and have mercy, for You are the best of those who show mercy!'') Here Allah is teaching us to recite this supplication, for forgiveness, in a general sense, means wiping away sins and concealing them from people, and mercy means guiding a person and helping him to say and do good things.