ALWAYS ASK FOR ALLAH FORGIVENESS.

Sunan an-Nasa'i, The Book of Seeking Refuge with Allah
Book 50, Hadith 95
Narrated 'It was narrated from Shaddad bin Aws that:
The Prophet [SAW] said: "The best of prayers for forgiveness is for a person to say: 'Allahumma, anta rabbi, la ilaha illa anta, khalaqtani wa ana 'abduka, wa ana 'ala 'ahdika wa wa'dika mastata'tu, a'udhu bika min sharri ma sana'tu, abuw'u laka bidhanbi, wa abuw'u laka bini'matika 'alayya faghfirli, fa innahu la yaghfirudh-dhunuba illa anta (O Allah, You are my Lord, there is no god but You. You have created me and I am Your slave and I am keeping my promise and covenant to You as much as I can. I seek refuge with You from the evil of what I do. I acknowledge Your blessing and I acknowledge my sin, so forgive me, for there is none who can forgive sin except You.)' If he says this in the morning, believing in it firmly, and dies on that day before evening comes, he will enter Paradise, and if he says it in the evening, believing firmly in it, and dies before morning comes, he will enter Paradise." Al-Walid bin Tha'labah contradicted him. (Sahih)

Sunan an-Nasa'i, The Book of Seeking Refuge with Allah
Book 50, Hadith 96
Narrated 'It was narrated from 'Abdah bin Abi Lubabah that Ibn Yasaf told him that he asked 'Aishah, the wife of the Prophet [SAW],:
What supplication did the Messenger of Allah [SAW] say the most before he died? She said: "The supplication that he said the most was: 'Allahumma, inni a'udhu bika min sharri ma 'amiltu wa min sharri ma lam a'mal ba'd (O Allah, I seek refuge with You from the evil of what I have done, and from the evil of what I have not done yet.)'" (Sahih)