Musnad Abu Ya'la al-Mawsili 159:142

Abu Musa Muhammad ibn al-Muthanna narrated to us, Salim ibn Nuh narrated to us, Yunus ibn Ubayd narrated to us, from Zurarah ibn Awfa, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah has pardoned for my nation what their souls whisper to them, as long as they do not act upon it or speak of it."