Musnad Ahmad 30:2037

Muhammad ibn Sabiq narrated to us, Sharik narrated to us, from al-A'mash, from Abu Salih, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, Allah, the Almighty and Majestic, wrote a book with His Hand for Himself before He created the heavens and the earth, and He placed it under His Throne. In it, it says: 'My mercy has preceded My wrath.'"