(O Allah! You are my Lord! None has the right to be worshipped but You. You created me and I am Your slave, and I am faithful to my covenant and my promise as much as I can. I seek refuge with You from all the evil I have done. I acknowledge before You all the blessings You have bestowed upon me, and I confess to You all my sins. So I entreat You to forgive my sins, for nobody can forgive sins except You.)
Shaddad ibn Aws (radiAllahu anhu) relates that the Prophet (salAllahu alayhi wasalam) said that he (Sayyid al-Istighfar) most superior way of asking for forgiveness from Allah is to say (the above du'a). That "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."
[sahih al-Bukhari; 8,75,318, at-Tirmidhi; 3393, an-Nasa'i; 5522, Ahmad; 16662]