It is narrated from Sayyiduna Uthman that I heard the Prophet (peace and blessings be upon him) say: Fasting is a shield just like you use a shield in the battlefield. And I heard the Prophet (peace and blessings be upon him) say: The best fast is three days in every month.
Hadith Referenceمسند احمد / مسند المدنيين رضي الله عنهم اجمعين / 16278