The Prophet (ﷺ) said, "The best supplication for seeking forgiveness (Syed-ul- Istighfar) is to say: '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 'alayya, wa abu'u bidhanbi faghfir li, fa innahu la yaghfirudh-dhunuba illa Anta. (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).' He who supplicates in these terms during the day with firm belief in it and dies on the same day (before the evening), he will be one of the dwellers of Jannah; and if anyone supplicates in these terms during the night with firm belief in it and dies before the morning, he will be one of the dwellers of Jannah."

[Al-Bukhari].