Musnad Ahmad 30:2042

Sulayman narrated to us, Isma'il narrated to us, al-'Ala' informed me, from his father, from Abu Hurayrah, that the Prophet, may Allah bless him and grant him peace, said: "If the believer knew what Allah, the Almighty and Majestic, has of punishment, no one would covet His Paradise. And if the disbeliever knew what Allah has of mercy, no one would despair of His mercy."