The Prophet ﷺ said, "Whoever fasts Ramadan, that month counts as ten months, and fasting six days after the breaking of the fast completes the fasting of the entire year."
حدثنا الحكم بن نافع حدثنا ابن عياش عن يحيى بن الحارث الذماري عن أبي أسماء الرحبي
عن النبي ﷺ قال من صام رمضان فشهر بعشرة أشهر وصيام ستة أيام بعد الفطر فذلك تمام صيام السنة
No references are available.
Its isnad is sahih. Yahya bin Harith Dhamari Shami is trustworthy, and his hadith appears in the Sunan. Tabarani related the hadith in 4/134, and Haythami did not attribute it to Ahmad in 3/184, saying, "Its narrators are the narrators of the Sahih." The hadith about six days of Shawwal has appeared many times before. See 12414, 14414, and 14645.