It is narrated from Sayyiduna Abu Ayyub Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever fasts in Ramadan and then follows it with six days of fasting in Shawwal, it is as if he has fasted for the whole year."
Hadith Referenceسنن دارمي / من كتاب الصوم / 1792
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده حسن ولكن الحديث صحيح، [مكتبه الشامله نمبر: 1795]
Hadith Takhrijاس روایت کی سند حسن لیکن حدیث صحیح ہے۔ دیکھئے: [مسلم 1164] ، [أبوداؤد 2433] ، [ترمذي 759] ، [ابن ماجه 1716] ، [ابن حبان 3634] ، [الحميدي 385] و [مجمع الزوائد 5177]
It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: “(The fasting of a year is like this:) the fasting of one month is equal to ten months, and after that, the fasting of six days is equal to two months, thus making twelve months and completing a whole year.” That is, one month of Ramadan and after it, six fasts of Shawwal.