Sahih Muslim Book 1, Hadith Number 230.

Chapter : Allah disregards the premonition of the evil promptings of the heart so long as they do not take a firm-root.

It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) observed: Verily Allah forgave my people the evil promptings which arise within their hearts as long as they did not speak about them or did not act upon them.