Musnad Ahmad 3:76

Surayj narrated to us, Ibn Abi al-Zinad narrated to us, from his father, from Aban ibn Uthman, who said: I heard Uthman ibn Affan saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever says at the beginning of his day, or at the beginning of his night: 'In the name of Allah, with whose name nothing on earth or in heaven can cause harm, and He is the All-Hearing, the All-Knowing,' three times, nothing will harm him during that day or during that night."