Musnad Ahmad 30:352

Yazid narrated to us, Mis'ar informed us, from Qatadah, from Zurarah ibn Awfa, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "My nation is pardoned for what occurs within themselves, or what their selves whisper to them, as long as they do not act upon it or speak of it."