Musnad Ahmad 30:2371

Isma'il narrated to us, from Sa'id, from Qatadah, from Zurarah, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, Allah, the Almighty and Majestic, has pardoned for my nation what their souls whisper to them, as long as they do not speak of it."