Musnad Ahmad 30:3679

Mu'ammal narrated to us, Hammad narrated to us, 'Asim narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Verily, Allah, the Almighty and Majestic, has one hundred mercies. He placed one mercy of them in the world, by which you show mercy to one another, and He has ninety-nine mercies with Him. So when the Day of Resurrection comes, He will join this mercy to the ninety-nine mercies, then He will bestow them upon His creation."