Hadith: O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I shall forgive you

عَنْ أَنَسِ بْنِ مَالِكٍ رضي الله عنه قَالَ:
سَمِعْتُ رَسُولَ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «قَالَ اللَّهُ تَبَارَكَ وَتَعَالَى: يَا ابْنَ آدَمَ إِنَّكَ مَا دَعَوْتَنِي وَرَجَوْتَنِي غَفَرْتُ لَكَ عَلَى مَا كَانَ فِيكَ وَلاَ أُبَالِي، يَا ابْنَ آدَمَ لَوْ بَلَغَتْ ذُنُوبُكَ عَنَانَ السَّمَاءِ ثُمَّ اسْتَغْفَرْتَنِي غَفَرْتُ لَكَ، وَلاَ أُبَالِي، يَا ابْنَ آدَمَ إِنَّكَ لَوْ أَتَيْتَنِي بِقُرَابِ الأَرْضِ خَطَايَا ثُمَّ لَقِيتَنِي لاَ تُشْرِكُ بِي شَيْئًا لأَتَيْتُكَ بِقُرَابِهَا مَغْفِرَةً».
[حسن] - [رواه الترمذي] - [سنن الترمذي: 3540]

Anas ibn Mālik (may Allah be pleased with him) reported:
I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "Allah Almighty said: 'O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I shall forgive you. O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to meet Me, ascribing no partner to Me, I shall bring you forgiveness nearly as great as it is.'"
[Good hadith] - [Narrated by At-Termedhy] - [Sunan At-Termedhy - 3540]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed that Allah Almighty said in a Qudsi Hadīth: O son of Adam, as long as you supplicate Me and hope for My mercy and do not despair, I will conceal your sin and eliminate it, not minding about that, even if the sin is one of the major ones. O son of Adam, if your sins grow so numerous that they fill what is between the heavens and the earth, extending over its different corners and areas, and then you ask forgiveness of Me, I shall erase and forgive all of them, not caring about how numerous they are.
O son of Adam, if you come to Me after death with sins and misdeeds nearly as great as the earth, and you have died as a monotheist, not associating anything with Me, I will meet these sins and misdeeds with forgiveness as great as the earth, for I am vast in forgiveness and I forgive all sins except for Shirk.