Musnad Ahmad 30:1579

Husayn narrated to us, Ibn Abi al-Zinad narrated to us, from his father, from al-A'raj, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "When Allah, the Almighty and Majestic, finished the creation, He wrote in His Book—and it is with Him above the Throne—'Verily, My mercy prevails over My wrath.'"