Jami` at-Tirmidhi: Book 48, Hadith 24

Shaddad bin Aws narrated that: The Prophet (ﷺ) said to him: “Should I not direct you to the chief of supplications for forgiveness? ‘O Allah, You are my Lord, there is none worthy of worship except You, You created me and I am Your slave. I am adhering to Your covenant and Your promise as much as I am able to, I seek refuge in You from the evil of what I have done. I admit to You your blessings upon me, and I admit to my sins. So forgive me, for there is none who can forgive sins except You (Allāhumma anta rabbī lā ilāha illā anta, khalaqtanī wa ana `abduka, wa ana `alā `ahdika wa wa`dika ma-staṭa`tu. A`ūdhu bika min sharri ma ṣana`tu, wa abū’u ilayka bini`matika `alayya wa a`tarifu bidhunūbī faghfirlī dhunūbī innahu lā yaghfirudh-dhunūba illā ant).’ None of you says it when he reaches the evening, and a decree comes upon him before he reaches morning, except that Paradise becomes obligatory upon him. And none says it when he reaches the morning, and a decree comes upon him before he reaches evening, except that Paradise becomes obligatory for him.”