I have committed many sins and want to sincerely repent to Allah. What is the most excellent and comprehensive supplication (Dua) for seeking forgiveness from Allah mentioned in the Sunnah?
In the name of Allah, the Most Gracious, the Most Merciful.
The doors of repentance are always open, and Allah is the Most Forgiving. The most excellent and comprehensive supplication for seeking forgiveness is known as Sayyid al-Istighfar (The Chief of Prayers for Forgiveness). The Prophet (ﷺ) stated that whoever recites this with firm faith during the day and dies on that day will enter Paradise, and the same applies to the night.
Reference:
اللَّهُمَّ أَنْتَ رَبِّي لا إِلَهَ إِلا أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ، وَأَبُوءُ لَكَ بِذَنْبِي فَاغْفِرْ لِي، فَإِنَّهُ لا يَغْفِرُ الذُّنُوبَ إِلا أَنْتَ
Allahumma anta Rabbi la ilaha illa anta, Khalaqtani wa ana ‘Abduka, wa ana ‘ala ‘ahdika wa wa’dika mastata’tu, A’udhu bika min Sharri ma sana’tu, abu’u Laka bini’matika ‘alaiya, wa Abu’u Laka bidhanbi faghfirli fa innahu la yaghfirudh-dhunuba illa anta.
Translation: “O Allah, You are my Lord, there is none worthy of worship but You. You created me and I am Your slave. I keep Your covenant and my pledge to You so far as I am able. I seek refuge in You from the evil of what I have done. I admit to Your blessings upon me, and I admit to my misdeeds. Forgive me, for there is none who may forgive sins but You.”
— [Sahih al-Bukhari: 6306]
May Allah accept your repentance and grant you steadfastness. And Allah knows best.