Musannaf Ibn Abi Shayba 4:25

Isma'il ibn Ibrahim narrated to us, from Muhammad ibn Ishaq, from Sa'id ibn Abi Hind, who said: Mutarrif ibn 'Abdullah ibn al-Shikhkhir narrated to us, saying: I came to 'Uthman ibn Abi al-'As, and he invited me to [drink] the milk of a she-camel. I said, "I am fasting." He said, "As for me, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Fasting is a shield against the Fire, like the shield of one of you in battle. And it is good to fast three days of every month.'"