Musnad Abu Ya'la al-Mawsili 159:28

Abu Khaythama narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, from the Prophet (may Allah bless him and grant him peace), who said: Allah said: "I have prepared for My righteous servants what no eye has seen, and no ear has heard, and has not occurred to the heart of any human being." The confirmation of that is in the Book of Allah: "No soul knows what has been hidden for them of comfort for eyes as reward for what they used to do."