From Sahih Bukhari (Hadith 4780):
Narrated Abu Huraira:The Prophet, said, "Allah said, 'I have prepared for My pious worshipers such things as no eye has ever seen, no ear has ever heard of, and nobody has ever thought of. All that is reserved, besides which, all that you have seen, is nothing." Then he recited:-- 'No soul knows what is kept hidden (in reserve) for them of joy as a reward for what they used to do
Reference: Bukhari 4780