The Book of Forgiveness

Riyad as-Salihin, The Book of Forgiveness
Book 20, Hadith 1873
Narrated 'Ibn 'Abbas (ra):
The Messenger of Allah (pbuh) said, "If anyone constantly seeks pardon (from Allah), Allah will appoint for him a way out of every distress and a relief from every anxiety, and will provide sustenance for him from where he expects not."

[Abu Dawud].

Riyad as-Salihin, The Book of Forgiveness
Book 20, Hadith 1874
Narrated 'Ibn Mas'ud (ra):
The Messenger of Allah (pbuh) said, "He who says: 'Astaghfir ullah-alladhi la ilaha illa Huwal-Haiyul-Qayyumu, wa atubu ilaihi (I seek the forgiveness of Allah, there is no true god except Allah, the Ever-Living, the Self- Subsisting, and I turn to Him in repentance),' his sins will be forgiven even if he should have run away from the battlefield (while he was engaged in fighting for the Cause of Allah)."

[Abu Dawud, At-Tirmidhi and Al-Hakim (on conditions of Al-Bukhari and Muslim for accepting Hadith)].

Riyad as-Salihin, The Book of Forgiveness
Book 20, Hadith 1875
Narrated 'Shaddad bin Aus (ra):
The Prophet (pbuh) said, "The best supplication for seeking forgiveness (Syed-ul- Istighfar) is 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, abu'u laka bini'matika 'alayya, wa abu'u bidhanbi faghfir li, fa innahu la yaghfirudh-dhunuba illa Anta. (O Allah! You are my Rubb. There is no true god except You. You have created me, and I am Your slave, and I hold to Your Covenant as far as I can. I seek refuge in You from the evil of what I have done. I acknowledge the favours that You have bestowed upon me, and I confess my sins. Pardon me, for none but You has the power to pardon).'

He who supplicates in these terms during the day with firm belief in it and dies on the same day (before the evening), he will be one of the dwellers of Jannah; and if anyone supplicates in these terms during the night with firm belief in it and dies before the morning, he will be one of the dwellers of Jannah."

[Al-Bukhari].