Sunan An-Nasa'i

It was narrated from Saeed bin Abi Hind that Mutarrif a man from Banu 'Amir bin Sa'sa'ah told him that:'Uthman bin Abi Al-As called for milk to be given to him (Mutarrif) to drink. Mutarrif said: 'I heard the Messenger of Allah say: 'Fasting is a shield like the shield of any one of you in battle.""' Abu Hurairah said: ""The Messenger of Allah said: 'Fasting is a shield.""' (Sahih)