Fasting six days of Shawwaal
Narrated Abu Ayyub:
The Prophet (sallallaahu alayhi wa sallam) as saying: If anyone fasts during Ramadan, then follows it with six days in Shawwaal, it will be like a perpetual fast.
[Sunan Abi Dawud Hadith No. 2433]
Ibn Qudaamah says in al-Mughni that Maalik disliked for a person to fast the six days of Shawwaal after the month of Ramadaan and said: I have not seen anyone from the scholars of fiQh keep these fasts [of Shawwaal] and nothing has reached me in regards to it from anyone of the Salaf. The people of knowledge dislike that and fear it will become a bid'ah and that which is not part of Ramadaan will be connected to it.
You will find in the Maaliki book al-Kaafi fi fiQh ahl al-Madiinah:
وأنكر صيام ست من صدر شوال إنكارا شديدا
[Maalik] criticized the [fasting] of the six days of Shawwaal with severe criticism.
It has also been narrated from Abu Hanifah that he disliked fasting the six days from Shawwaal.
The chosen opinion of the Hanafi school, however, is that there is no harm in fasting these six days. They explain Abu Hanifah's verdict as to mean that the dislike is only so people do not start to think it is a part of the month and end up resembling the Nasaaraa.
Since that fear is now no longer present, it being disliked to fast in these six days is no longer applicable.
Another understanding for what Abu Hanifah said, which was presented by الكاساني is that it is only disliked if someone fasts one day on the day of fiTr and then fasts five days after it. That is what is disliked. Otherwise, if someone fasts after not fasing on the day of 'iid then it is not disliked. Rather, it is Mustahabb and a Sunnah.
The Maaliki school has also explained Imaam Maalik's dislike for the six days of Shawwaal in a similar manner.
قال الحطاب: قال في المقدمات: كره مالك - رحمه الله تعالى - ذلك مخافة أن يلحق برمضان ما ليس منه من أهل الجهالة والجفاء، وأما الرجل في خاصة نفسه فلا يكره له صيامها.
[Paraphrase] Imaam Maalik disliked that fearing that something which is not a part of Ramadaan will be made a part of it by the ignorant people. As for a person, when refering to him sepcifically, it is not disliked for him to fast the six days of Shawwaal.
However, it should be pointed out that the Maaliki school says that the virtue of fasting six days in Shawwaal in not limited to only Shawwaal. One could fast it in another month and get the same reward. The mention of Shawwaal by the Shaari' is for takhfiif in relation to the fast and not because of takhSiiS for that time.
Hence, according to them the farther away from Ramadaan when you keep these six fasts the more reward you will get because of the increased difficulty you will have to face in keeping them [i.e. it is easier to fast six right after fasting 30 days of Ramadaan, then it is to fast those six a few months later when you are no longer in the habit of fasting].
Reference(s):
al-Mughni li Ibn Qudaamah
Volume 3 Page 176
The Kuwaiti Encyclopedia of fiQh
Volume 28 Page 93
al-Kaafi fi fiQh ahl al-Madinah
Volume 1 Page 350