Hadith: Allah has written down the good deeds and the bad ones. Then, He explained that: He who intends a good deed and does not do it, Allah writes it down with Himself as a full good deed; and if he intends it and does it, Allah writes it down with Himself as ten good deeds to seven hundred times, or many times over. Yet, if he intends a bad deed and does not do it, Allah writes it down with Himself as a full good deed. But, if he intends it and does it, Allah writes it down as one bad deed

عن ابن عباس رضي الله عنهما
عَنِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فِيمَا يَرْوِي عَنْ رَبِّهِ عَزَّ وَجَلَّ قَالَ: قَالَ: «إِنَّ اللهَ كَتَبَ الْحَسَنَاتِ وَالسَّيِّئَاتِ، ثُمَّ بَيَّنَ ذَلِكَ، فَمَنْ هَمَّ بِحَسَنَةٍ فَلَمْ يَعْمَلْهَا كَتَبَهَا اللهُ لَهُ عِنْدَهُ حَسَنَةً كَامِلَةً، فَإِنْ هُوَ هَمَّ بِهَا فَعَمِلَهَا كَتَبَهَا اللهُ لَهُ عِنْدَهُ عَشْرَ حَسَنَاتٍ إِلَى سَبْعِمِائَةِ ضِعْفٍ، إِلَى أَضْعَافٍ كَثِيرَةٍ، وَمَنْ هَمَّ بِسَيِّئَةٍ فَلَمْ يَعْمَلْهَا كَتَبَهَا اللهُ لَهُ عِنْدَهُ حَسَنَةً كَامِلَةً، فَإِنْ هُوَ هَمَّ بِهَا فَعَمِلَهَا كَتَبَهَا اللهُ لَهُ سَيِّئَةً وَاحِدَةً».
[صحيح] - [متفق عليه] - [صحيح البخاري: 6491]

Ibn ‘Abbās (may Allah be pleased with him) reported:
The Prophet (may Allah's peace and blessings be upon him), as part of what he narrated from his Lord, said: "Allah has written down the good deeds and the bad ones. Then, He explained that: He who intends a good deed and does not do it, Allah writes it down with Himself as a full good deed; and if he intends it and does it, Allah writes it down with Himself as ten good deeds to seven hundred times, or many times over. Yet, if he intends a bad deed and does not do it, Allah writes it down with Himself as a full good deed. But, if he intends it and does it, Allah writes it down as one bad deed."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 6491]

Explanation

The Messenger (may Allah's peace and blessings be upon him) points out that Allah predestined the good deeds and the bad ones and then clarified to the two angels how to write them down:
If a person wants, intends, and resolves to do a good deed, it will be written for him as one good deed, even if he has not done it, and if he does it, its reward gets multiplied from ten to seven hundred times and many times over. The increase depends on the sincerity within the person's heart, how much benefit his good deed brings to others, and so on.
On the other hand, if a person wants, intends, and resolves to do a bad deed and then abandons it for Allah's sake, it will be written for him as a good deed. Yet, if he abandons it due to preoccupation with something else, while he has not engaged in the means leading to it, it will not be written as anything. But if he abandons it out of inability, his intention will be written against him. And if he does it, it will be written as one bad deed.