Musnad Ahmad 30:2748

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from 'Asim ibn Bahdalah, from Dhakwan, from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, that they said: "O Messenger of Allah, one of us thinks of something that he would hate to speak about, even if he were to have everything on earth." He said: "That is pure faith."