Hadith 34, 40 Hadith Qudsi
On the authority of Anas (may Allah be pleased with him), who said:
I heard the Messenger of Allah (ﷺ) say: Allah the 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 would 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 face Me, ascribing no partner to Me, I would bring you forgiveness nearly as great as it.

It was related by at-Tirmidhi (also by Ahmad ibn Hanbal). Its chain of authorities is sound.
عَنْ أَنَسٍ رَضِيَ اللهُ عَنْهُ ،  قَالَ : سَمِعْتُ  رَسُولَ اللهِ  صَلَّى الله عَلَيْهِ وَ سَلَّمَ ، يَقُولُ :  
  
قَالَ اللهُ تَعَالَى : يَا ابْنَ ادَمَ ، إِنَّكَ مَا دَعَوْتََنِي وَرَجَوْتَنِي ، غَفَرْتُ لَكَ عَلَى مَا كَانَ مِنْكَ وَلَا أُبالِي . يا ابْنَ ادَمَ  :لَوْ بَلَغَتْ ذُنُوبُكَ عَنانَ السَّماءِ ثُم َّ اسْتَغْفَرْتَني ، غَفَرْتُ لَكَ . يَا ابْنَ ادَمَ : إِنَّكَ لَوْ أَتَيْتَنِي بِقُرَابِ الأَرْضِ خَطَايا ثُمَّ لَقِيتَني لَا تُشْرِكُ بِي شَيْأً ، لَأَتيْتُكَ بِقُرَابِها مَغْفِرَةً 
    
رواهُ الترمذي (وكذلك أحمد) وسنده حسن