Sunan An-Nasa'i

It was narrated that Mutarrif said:""I entered upon 'Uthman bin Abi Al-As and he called for milk. I said: I am fasting; He said: ""I heard the Messenger of Allah say: 'Fasting is a shield like the of any one of you in battle.""' Abu Hurairah said: ""The Messenger of Allah said: 'Fasting is a shield.""' (Sahih)