Mutarrif reports: Sayyiduna Uthman bin Abu al-As (may Allah be pleased with him) asked for some milk to be brought so that he could give it to me to drink. Mutarrif said: I said, "I am fasting." So Sayyiduna Uthman (may Allah be pleased with him) said: I heard the Prophet (peace and blessings of Allah be upon him) say: "Fasting is a shield, just as a shield is for a person when participating in battle." I heard the Prophet (peace and blessings of Allah be upon him) say: "The best fasts are three days of every month."
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3649
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «التعليق الرغيب» (2/ 60).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم