Sayyiduna Thawban (may Allah be pleased with him), who was the freed slave of the Prophet Muhammad (peace and blessings be upon him), narrates this statement of the Prophet Muhammad (peace and blessings be upon him): "Whoever fasts the fasts of Ramadan and then fasts six days of Shawwal, it is as if he has fasted for the whole year."
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3635
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح: م - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح