Musnad Abu Ya'la al-Mawsili 159:141

Hadaba ibn Khalid narrated to us, Hammam narrated to us, from Qatada, from Zurara ibn Awfa, from Abu Huraira, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah has pardoned for my Ummah what their souls whisper to them, as long as they do not speak of it or act upon it."