Musnad Ahmad 572:13

'Affan narrated to us, Hammad bin Salamah narrated to us, Sa'id al-Jurairi informed us, from Abu al-'Ala', from Mutarrif, who said: I entered upon 'Uthman bin Abi al-'As, and he ordered for me the milk of a milch camel. I said: "I am fasting." He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Fasting is a shield against the punishment of Allah, like the shield of one of you in battle.'" "And fasting three days of every month is good."