كتاب الإيمان
1  
The Book of Faith
(59)
Chapter: If a person thinks of doing a good deed it will be recorded for him, and if he thinks of doing a bad deed it will not be recorded for him
(59)
باب إِذَا هَمَّ الْعَبْدُ بِحَسَنَةٍ كُتِبَتْ وَإِذَا هَمَّ بِسَيِّئَةٍ لَمْ تُكْتَبْ ‏‏
Sahih Muslim 130

It is narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) observed:

He who intended to do good, but did not do it, one good was recorded for him, and he who intended to do good and also did it, ten to seven hundred good deeds were recorded for him. And he who intended evil, but did not commit it, no entry was made against his name, but if he committed that, it was recorded.
وَحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ، عَنْ هِشَامٍ، عَنِ ابْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مَنْ هَمَّ بِحَسَنَةٍ فَلَمْ يَعْمَلْهَا كُتِبَتْ لَهُ حَسَنَةً وَمَنْ هَمَّ بِحَسَنَةٍ فَعَمِلَهَا كُتِبَتْ لَهُ عَشْرًا إِلَى سَبْعِمِائَةِ ضِعْفٍ وَمَنْ هَمَّ بِسَيِّئَةٍ فَلَمْ يَعْمَلْهَا لَمْ تُكْتَبْ وَإِنْ عَمِلَهَا كُتِبَتْ ‏"‏ ‏.‏
Reference : Sahih Muslim 130
In-book reference : Book 1, Hadith 244
USC-MSA web (English) reference : Book 1, Hadith 236
  (deprecated numbering scheme)