Hadith 42, 40 Hadith an-Nawawi
On the authority of Anas (may Allah be pleased with him) who said:
I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, “Allah the Almighty has said: ‘O Son of Adam, as long as you invoke 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 you then asked forgiveness from 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 [too].’ ” 

It was related by at-Tirmidhi, who said that it was a hasan hadeeth.
عَنْ أَنَسِ بْنِ مَالِكٍ رَضِيَ اللهُ عَنْهُ قَالَ: سَمِعْت رَسُولَ اللَّهِ صلى الله عليه و سلم يَقُولُ: قَالَ اللَّهُ تَعَالَى: "يَا ابْنَ آدَمَ! إِنَّكَ مَا دَعَوْتنِي وَرَجَوْتنِي غَفَرْتُ لَك عَلَى مَا كَانَ مِنْك وَلَا أُبَالِي، يَا ابْنَ آدَمَ! لَوْ بَلَغَتْ ذُنُوبُك عَنَانَ السَّمَاءِ ثُمَّ اسْتَغْفَرْتنِي غَفَرْتُ لَك، يَا ابْنَ آدَمَ! إنَّك لَوْ أتَيْتنِي بِقُرَابِ الْأَرْضِ خَطَايَا ثُمَّ لَقِيتنِي لَا تُشْرِكُ بِي شَيْئًا لَأَتَيْتُك بِقُرَابِهَا مَغْفِرَةً" . 
رَوَاهُ التِّرْمِذِيُّ [رقم:3540]، وَقَالَ: حَدِيثٌ حَسَنٌ صَحِيحٌ.