Seek your Lord’s forgiveness and turn to Him in repentance, for my Lord is indeed Most Merciful and Loving. (al-Qur’an 11:90)
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THE PROPHET 
said: ‘Allah, Blessed and Exalted is He, says, ‘O son of Adam, as long as you call on Me and ask from Me, I shall forgive you for what you have done, and think nothing of it.
O son of Adam, even if your sins were to reach up to the clouds in the sky, and then you were to ask for My forgiveness, I would forgive you and think nothing of it.
O son of Adam, even if you were to come to Me with sins nearly as great as the earth, and then you were to meet Me (i.e. after death), not worshipping anything besides Me, I would bring you forgiveness nearly as great as the earth’. (Tirmidhi)
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THE PROPHET said: ‘Turn to Allah (in repentance), O people, and ask for forgiveness (for your sins); for truly, I myself turn to Him in repentance one hundred times a day’. (Muslim)
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THE PROPHET said: ‘All Children of Adam are prone to make mistakes, but the best of those who do wrong are those who constantly repent’.3 (Ibn Majah, Tirmidhi)
THE PROPHET 
GAVE US MANY EXAMPLES OF HOW TO PRAY FOR FORGIVENESS. HERE ARE TWO SIMPLE PRAYERS:
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THE PROPHET said: ‘O Allah, forgive all my sins – great and small, first and last, open and secret’. (Muslim)
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IBN UMAR 
relates: ‘We used to witness the Messenger of Allah, repeating one hundred times in one session: ‘O Lord, forgive me and grant me repentance, for You are the Most Forgiving, the Most Merciful. (Abu Dawud)
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3. By making an intention not to repeat their sin, and seeking His forgiveness.