Shaddad ibn Aws narrated that the Prophet [صلى الله عليه وسلم] said; "the most superior request for forgiveness is to say:
'Allaahmma anta rabbee laa elaaha illaa ant. Anta khalaqtanee wa ana 'abduka wa ana 'alaa 'ahdika wa wa'dika mastata't. A'oothu bika min sharri ma sana't. Aboo'u laka bi ni'matika alayya wa aboo'u laka bi thambee. Faghfir lee. Fa innahoo laa yaghfiruth-thunooba illaa ant.'
[O Allah, You are my Lord. There is no god besides You. You created me and I am Your servant, following your covenant and [my] promise to you as much as I can. I seek refuge in You from the evil that I have done. Before You I acknowledge Your blessings bestowed upon me and I confess my sins to You. So forgive me, for surely no one can forgive sins except You.]
The Prophet [صلى الله عليه وسلم] then added, 'anyone who says this during the day, firmly believing in it, and dies before the evening; or says it in the evening, firmly believing in it, and dies before the following morning, will be among the people of paradise.' "
[Sahih al-Bukhari, vol 8, pp212-3, no 318]