(Sahih Muslim, Page 570, Hadith No. 2614/5/6)
(Ahmad 5. 280, Ibn Hibbaan 3627, Nasai Fil Kubra 2860)
Fasting in Ramadhan and following it with six fasts in Shawwaal is equal to fasting throughout the year because the reward of a good deed is multiplied ten times as also mentioned in the above Hadith of Thaubaan Radhiyallahu Anhu.
There is no difference whether Ramadhaan is of twenty-nine days or thirty days. So if a person follows Ramadhaan with six fasts in Shawwaal, it will equal fasting the entire year irrespective of whether Ramadhaan was of twenty-nine days or thirty days.
Someone could make the objection that if a person keeps these six fasts in any month apart from Shawwaal, he will still receive this reward. Why, then, is fasting specified in Shawwaal? The reply to this is that fasting in Shawwaal is attached to the virtue of fasting in Ramadhaan. The person will therefore receive the reward of keeping COMPULSORY fasts for a full year. This is stated by Ibn al Mubarak RA. It is stated that Imam At’ Tirmizi RA narrated it from him in a part of a Hadith. (Tirmizi 3/123)
Being in the habit of fasting after the fasts of Ramadhaan is a sign of acceptance of the Ramadhaan fasts. This is because when Allah Ta’ala accepts a deed from a servant, he enables him to follow it with a good deed.