Musannaf Ibn Abi Shaybah

Hazrat Mutarrif bin Abdullah bin Shikhir says that I once visited Hazrat Usman bin Abi Al-Aas. He had milk from a pregnant she-camel brought for me. I said that I am fasting. He said that I heard the Messenger of Allah, peace and blessings be upon him, say that fasting is a shield against Hell just as you take a shield against an enemy. The best way to fast is to keep three fasts every month.