Musnad Ahmad 30:1100

The Messenger of Allah, may Allah bless him and grant him peace, said: "The angels said: 'O Lord, that servant of Yours wants to commit a bad deed,' and He is more discerning of him. He said: 'Watch him. If he does it, then write it down for him as the like of it. And if he leaves it, then write it down for him as a good deed, for he only left it for My sake.'"