Musnad Abu Ya'la al-Mawsili 160:112

And with his chain of narration, that the Messenger of Allah (may Allah bless him and grant him peace) said: "If the believer knew what Allah, the Mighty and Majestic, has of punishment, no one would hope for His Paradise. And if the disbeliever knew what Allah has of mercy, no one would despair of His Paradise."