Musnad Ahmad 30:3150

'Abd al-Rahman narrated to us, he said: Zuhayr narrated to us, from al-'Ala', from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "If the believer knew what punishment is with Allah, no one would covet His Paradise. And if the disbeliever knew what mercy is with Allah, no one would despair of His Paradise." Allah created one hundred mercies, and He placed one among His creation by which they show mercy to one another, and with Allah are ninety-nine mercies.