In Sunan Ibn Majah it is narrated on the authority of Thauban (R.A.) that the Messenger (sallallahu alaihi wa sallam) said ““Whoever fasts for six days after (Eid) al-Fitr, it will be like the fasting of a year.” (Mirqaatul Mafatih: 4/545)
The scholars explain that the reason why fasting in Ramadan then fasting for six days in Shawwal is equal to fasting for a whole year is that each reward of a person is multiplied by ten. Therefore, fasting in Ramadan will be equal to fasting for 300 days (30 x 10 = 300); and fasting for an additional six days will be equal to fasting for 60 days (6 x 10 = 60). Fasting in Ramadan then for 6 days in Shawwal will altogether be equal to fasting for 360 days, which is the equivalent of one year. This is also supported by a hadith in Sunan Nisaa’i. (Sharh Muslim of Nawawi: 1/369; Ahsanul Fatawa: 4/440; Mirqaatul Mafatih: 4/544)
It thus becomes clear that the reward of fasting for a year is applicable to a person who keeps the fasts of Ramadan as well as six days of Shawwal. Therefore, it is not correct to say that if a person keeps 12 days of fasts in Shawwal instead of 6 days, he will get the reward of fasting for two years.
Mufti Faizal Riza
This answer was collected from Fatwa.org.au, which is connected to Darul Ifta Australia, based in Melbourne, Australia.
It is operated by Mufti Faizal Riza, a student of Mufti Ebrahim Desai from South Africa.