On the authority of Ibn Abbas (may Allah be pleased with him), from the Messenger of Allah (peace and blessings of Allah be upon him), from what he has related from his Lord:
Verily Allah ta'ala has written down the good deeds and the evil deeds, and then explained it [by saying]: "Whosoever intended to perform a good deed, but did not do it, then Allah writes it down with Himself as a complete good deed. And if he intended to perform it and then did perform it, then Allah writes it down with Himself as from ten good deeds up to seven hundred times, up to many times multiplied. And if he intended to perform an evil deed, but did not do it, then Allah writes it down with Himself as a complete good deed. And if he intended it [i.e., the evil deed] and then performed it, then Allah writes it down as one evil deed."[Al-Bukhari][Muslim].
Reference: 40 Hadith Nawawi 37
English translation: Hadith 37