Sahih al-Bukhari #6306Sahih
اللَّهُمَّ أَنْتَ رَبِّي لَا إِلَٰهَ إِلَّا أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ، وَأَبُوءُ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ
The Prophet (peace be upon him) said: 'The master supplication for forgiveness is to say: Allahumma Anta Rabbi...' He then said: 'Whoever says it during the day with conviction and dies that day before evening, he will be among the people of Paradise. And whoever says it during the night with conviction and dies before morning, he will be among the people of Paradise.'