Question: What is your legal opinion on fasting six days in Shawwaal after the end of Ramadaan? I read in Muwatta’ Imam Malik that Imaam Maalik ibn Anas said, regarding the fasting of the six days of Shawwaal, that he never saw one of the scholars or jurists fasting on these days. He was also never informed that any of the Salaf (righteous predecessors) did this. The scholars also regarded it as Makrooh (disliked) and feared that it might be an act of Bid‘ah (innovation in religion), and that these days might be wrongly considered as part of Ramadaan. This was stated in the Muwatta’, vol. 1, no. 228.
Answer: It was authentically reported on the authority of Aboo Ayyoob (may Allaah be pleased with him) that the Messenger of Allaah (peace be upon him) said: Anyone who fasts Ramadaan and then follows it with six days of Shawwaal it is as if they fasted the whole year. (Related by Ahmad, Muslim, Aboo Daawood, and Al-Tirmithee) This Hadeeth Saheeh (authentic Hadeeth) is evidence that fasting six days of Shawwaal is an act of Sunnah (supererogatory act of worship following the example of the Prophet), which was acted upon by Al-Shaafi‘ee, Ahmad,and a group of other eminent scholars... read more here.