The Supplication Asking For Forgiveness

 عَنْ أَبِي بَكْرٍ الصِّدِّيقِ رَضِيَ اللَّهُ عَنْهُ أَنَّهُ قَالَ لِلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلَاتِي قَالَ قُلْ اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ 

Abu Bakr As-Siddiq (may Allah be pleased with him) reported: I requested the Messenger of Allah (saw) to teach me a supplication which I could recite in my Salat (prayer). There upon he said, "Recite:  

اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ 

(O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate)." 

(Al-Bukhari and Muslim) 

Status: Sahih 

(Hadith translation taken from http://sunnah.com/riyadussaliheen/17/11) 

Lessons: 

1.  Supplication is a necessity for us as weak servants

2.  Two of the best times to supplicate are during sujood (prostration), with the supplication read in Arabic, and after performing obligatory prayer

3.  A supplication taught by the Messenger of Allah (pbuh) is one specifically asking for forgiveness for mistakes or wrongdoings, whether they were done consciously or not:

اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ 

فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ 

" O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate." 

Let us memorise and practice this supplication.