Prophet Muhammad (peace be upon him) said:

Allah, the Exalted and Glorious, Stretches out His Hand during the night so that the people may repent for the fault committed from dawn till dusk and He stretches out His Hand during the day so that the people may repent for the fault committed from dusk to dawn. (He would accept repentance) before the sun rises in the west (before the Day of Resurrection). (Muslim)

“If you were to commit sin until your sins reach the heaven, then you were to repent, your repentance would be accepted.” (Ibn Majah)

Allah is more pleased with the repentance of a servant as he turns towards Him for repentance than this that one amongst you is upon the camel in a waterless desert and there is upon (that camel) his provision of food and drink also and it is lost by him, and he having lost all hope (to get tbat) lies down in the shadow and is disappointed about his camel and there he finds that camel standing before him. He takes hold of his nosestring and then out of boundless joy says: 0 Lord, Thou art my servant and I am Thine Lord. He commits this mistake out of extreme delight. (Muslim)

Prayers from Quran

Our Lord, indeed we have heard a caller [i.e., Prophet Muhammad] calling to faith, [saying], ‘Believe in your Lord,’ and we have believed.  Our Lord so forgive us our sins and remove from us our misdeeds and cause us to die with the righteous. (Quran 3:193)

My Lord, forgive and have mercy, and You are the best of the merciful. (Quran 23:118)

Our Lord, we have wronged ourselves, and if You do not forgive us and have mercy upon us, we will surely be among the losers. (Quran 7:23)

The Best Prayer of Seeking Forgiveness

“O Allah, You are my Lord.  There is no true god besides You. You created me and I am your slave, following Your covenant and Your promise as much as I can.  I seek refuge in You from the evil I have done.  I acknowledge before You Your blessings bestowed on me and I confess to You my sins.  So forgive me, for surely, no one can forgive sins except You.” (Al-Bukhari)

Transliterated Text

“Allah-humma ’Anta Rabbi laa ’ilaha il-la ’Ant.  Khalaqtani wa ana ’abduka wa ana ’alaa ’ah-dika wa wa’dika mas-ta-ta’t.  ’A’odhu bika min sharri ma san’at.  ’Aboo’u la-ka bi ni’mati-ka ’alay, wa ’aboo’u bi dhanbi faghfir-lee fa ’in-nahu laa yaghfir udh-dhunuba ’il-la ’Ant.”