In the hadith of Bukhari The Prophet (ﷺ) is reported as saying that syed ul istighfar (sayyidul istighfar) is the the best supplication or the most superior way for a believer to seek forgiveness from Allah. 

The Prophet (ﷺ) added. “If somebody recites it during the day with firm faith in it, and dies on the same day before the evening, he will be from the people of Paradise; and if somebody recites it at night with firm faith in it, and dies before the morning, he will be from the people of Paradise.”

Reference: Sahih al-Bukhari 6306
In-book reference : Book 80, Hadith 3

allahumma anta rabbi in Arabic Text

In arabic the dua is written,

اللّٰهُمَّ أَنْتَ رَبِّي، لَا إِلٰهَ إِلَّا أَنْتَ، خَلَقْتَنِيْ وَأَنَا عَبْدُكَ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوْذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَىَّ وَأَبُوءُ لَكَ بِذَنْبِيْ، فَاغْفِرْ لِيْ، فَإِنَّهُ لَا يَغْفِرُ الذُّنُوْبَ إِلَّاأَنْتَ۔

Transliteration of syed ul istighfar

Here’s transliteration to help with pronunciation of the dua,

Allahumma anta rabbi la ilaha illa anta, khalaqtani wa-ana ‘abduk, wa-ana ‘ala ‘ahdika wawa‘dika mas-tata‘t, a‘oothu bika min sharri ma sana‘t, aboo-o laka bini‘matika ‘alay, wa-aboo-o bithanbee, faghfir lee fa-innahu la yaghfiru-thunaba illa ant

English Translation:

The meaning of the allahumma anta rabbi dua is, “O Allah! You are my Rubb. There is no true god except You. You have created me, and I am Your slave, and I hold to Your Covenant as far as I can. I seek refuge in You from the evil of what I have done. I acknowledge the favours that You have bestowed upon me, and I confess my sins. Pardon me, for none but You has the power to pardon”.

 Listen to audio version of the dua below.

The importance of asking for forgivness:

Asking for forgiveness increases your strength.

“And O my people, ask forgiveness of your Lord and then repent to Him. He will send [rain from] the sky upon you in showers and increase you in strength [added] to your strength. And do not turn away, [being] criminals.” Surah Hud Ayat 52

Allah forgives those who ask for forgiveness. There is a great reward to those who are humble, fast, and guard their privates:

“Indeed, the Muslim men and Muslim women, the believing men and believing women, the obedient men and obedient women, the truthful men and truthful women, the patient men and patient women, the humble men and humble women, the charitable men and charitable women, the fasting men and fasting women, the men who guard their private parts and the women who do so, and the men who remember Allah often and the women who do so – for them Allah has prepared forgiveness and a great reward.” Surah Ahzab ayat 35.