Narrated Shaddad bin Aus:
The Prophet said "The most superior way of asking for forgiveness
from Allah is: 'Allahumma anta Rabbi la ilaha illa anta, 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 'alaiya, wa Abu
Laka bidhanbi faghfirli innahu la yaghfiru adhdhunuba illa anta."
The Prophet 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."
Bukhari,Volume 8, Book 75, Number 318.