The Prophet (SAW) said: 'Allah, Blessed, Exalted is He, says, 'O son of Adam, as long as you call on Me and ask of 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'.
The Prophet (SAW) 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'.
The Prophet (SAW) said:'All Children of Adam are prone to make mistakes, but the best of those who wrong are those who constantly repent.'
Ibn Majah, Tirmidhi
How to Pray for Forgiveness
The Prophet (SAW) said: 'O Allah, forgive all my sins, great and small, first and last, open and secret'.
Ibn Umar (RA) relates:'We used to witness the Messenger of Allah repenting one hundred times in one session: 'O Lord forgive me and grant me repentance, for You are the Most forgiving, the Most Merciful.'