Sahih Muslim Book - 1, Hadith - 338

حَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ حَدَّثَنَا عَبْدُ الْوَارِثِ عَنْ الْجَعْدِ أَبِي عُثْمَانَ حَدَّثَنَا أَبُو رَجَاءٍ الْعُطَارِدِيُّ عَنْ ابْنِ عَبَّاسٍ عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِيمَا يَرْوِي عَنْ رَبِّهِ تَبَارَكَ وَتَعَالَى قَالَ إِنَّ اللَّهَ كَتَبَ الْحَسَنَاتِ وَالسَّيِّئَاتِ ثُمَّ بَيَّنَ ذَلِكَ فَمَنْ هَمَّ بِحَسَنَةٍ فَلَمْ يَعْمَلْهَا كَتَبَهَا اللَّهُ عِنْدَهُ حَسَنَةً كَامِلَةً وَإِنْ هَمَّ بِهَا فَعَمِلَهَا كَتَبَهَا اللَّهُ عَزَّ وَجَلَّ عِنْدَهُ عَشْرَ حَسَنَاتٍ إِلَى سَبْعِ مِائَةِ ضِعْفٍ إِلَى أَضْعَافٍ كَثِيرَةٍ وَإِنْ هَمَّ بِسَيِّئَةٍ فَلَمْ يَعْمَلْهَا كَتَبَهَا اللَّهُ عِنْدَهُ حَسَنَةً كَامِلَةً وَإِنْ هَمَّ بِهَا فَعَمِلَهَا كَتَبَهَا اللَّهُ سَيِّئَةً وَاحِدَةً.

It was narrated from Ibn 'Abbas that the Messenger of Allah (s.a.w) said, relating from his Lord, the Mighty and Sublime: "Allah decreed good deeds and bad deeds, then He explained that. Whoever thinks of doing a good deed then does not do it, Allah will write it down as one complete good deed. If he thinks of doing a good deed and then does it, Allah [the Mighty and Sublime] will write it down between ten and seven-hundred fold, or many more. If he thinks of doing a bad deed then he does not do it, Allah will write it down as one complete good deed, and if he thinks of it then does it, Allah will write it down as one bad deed."