Authentic Supplication Number 20.

It is reported of the Prophet  that he said: “The master of invocations for forgiveness is: ‘Allahumma ‘anta rabbee, la ‘ilaha ‘illa ‘anta, khalaqtanee, wa ‘ana `abduka, wa ‘ana `ala `ahdika wa wa`dika mastata`tu, ‘a`outhu bika min sharri ma sana`tu, ‘aboo’u laka bini`matika ‘alayya, wa ‘aboo’u bithambee, faghfirlee fa’innahu la yaghfiru ‘aththunooba ‘illa ‘anta (37).

Whoever says this as he enters upon evening, then, dies that night, he would enter Paradise; and if one says this as he enters upon morning, then, dies that day, he would enter Paradise”.

Footnotes.
(37) O’ my ‘ilah You are my Lord, there is no ‘ilah but You. You created me, and I am your bondservant, and I will stick to my covenant and promise [of faith and sincere obedience] to You, as to my ability. I seek refuge in You from the evil of what I have done, I acknowledge, to You, your bounties upon me, and I acknowledge, to You, my sin. Thus forgive me, for none forgives sins except You.

Reporters.
Reported by Al-Bukhari. Al-Albani included this hadeeth in The Authentic of Good Sayings as #20.