Musnad Ahmad 572:6

Hashim narrated to us, he said: Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Sa'id ibn Abi Hind, that Mutarrif, a man from Banu 'Amir ibn Sa'sa'ah, narrated to him that 'Uthman ibn Abi al-'As al-Thaqafi offered him milk to drink. Mutarrif said: "I am fasting." 'Uthman said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Fasting is a shield from the Fire, like the shield of one of you in battle.'"