Musnad Ahmad 299:11

Hajjaj narrated to us, he said: Layth ibn Sa'd narrated to us, he said: Yazid ibn Abi Habib narrated to me, from Sa'id ibn Abi Hind, that Mutarrif, from the tribe of Banu 'Amir ibn Sa'sa'ah, narrated to him that 'Uthman ibn Abi al-'As al-Thaqafi called for milk for him 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 against the Fire, like the shield of one of you in battle.'"