Musnad Ahmad 30:1296

Abu Amir narrated to us, 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 hope for Paradise. And if the disbeliever knew what mercy is with Allah, no one would despair of Paradise. Allah created one hundred mercies, and He placed one mercy among His creation by which they show mercy to one another, and with Allah are ninety-nine mercies."