Sahih Bukhari Volume 008, Book 075, Hadith Number 335.
Narated By Shaddad bin 'Aus : The Prophet said, "The most superior way of asking for forgiveness from Allah is: 'Allahumma anta Rabbi la ilaha illa anta. Khalaqtani wa ana 'abduka, wa ana 'ala 'ahdika wa Wa'dika mastata'tu abu'u Laka bi ni 'matika wa abu'u Laka bidhanbi; faghfirli fa'innahu la yaghfiru-dh-dhunuba ill a ant a. A'uidhu bika min sharri ma sana'tu.' If somebody recites this invocation during the night, and if he should die then, he will go to Paradise (or he will be from the people of Paradise). And if he recites it in the morning, and if he should die on the same day, he will have the same fate."