بسم الله الرحمن الرحيم
Upon the authority of Aboo Bakr as-Siddeeq, may Allaah be pleased with him, that he once asked the Prophet, may Allaah's Salaah and Salaam be upon him;
"Teach me a Du'aa that I can supplicate with in my prayer."
He, may Allaah’s Salaah and Salaam be upon him, said, “Say:
اللهُمَّ إنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيْرًا وَ لَا يَغْفِرُ الذُّنُوْبَ إِلَا أَنْتَ
فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَ ارْحَمْنِي
إِنَّكَ أَنْتَ الغَفُوْرُ الرَّحِيْمُ
O Allaah! Indeed I have severely wronged myself and none forgives sins except You. So grant me Your Forgiveness and have Mercy on me, verily You are the Most-Forgiving, the Bestower of Mercy.
Reported by Bukhaaree and Muslim.