Allaamah Abdul Wahhaab Sha’raani (Rahmatullah alayh) who was among the greatest Fuqaha, Muhadditheen and Auliya of the 10th Islamic century, says in his Kitaab, Lawaaqihul Anwaaril Qudsiyya:
“A universal pledge emanating from Rasulullah (Sallallahu alayhi wasallam) has been taken from us that we should follow up the Saum of Ramadhaan with six days of fasting in the month of Shawwaal, in order to purify us from the sin of impurities which are the effects of negligence on the Day of Eid, in the wake of gluttonous feasting from which the nafs was restrained by fasting during the month of Ramadhaan.
Frequently the nafs advances recklessly with lustful eating on the Day of Eid. In consequence (of this gluttony) develop such ghaflat (obliviousness) and hijaab (spiritual deprivation) which is worse than the total lusts which are restrained as a result of the month-long abstention during Ramadhaan. (So deadly is the effect of the gluttony and fulfilment of lust on the Day of Eid).
Therefore these Six fasts of Shawwaal are an expiation for the deficiencies in our fasting of Ramadhaan. They are like the Sunnat Salaat which accompanies the Fardh Salaat or like Sajdah Sahw (which rectifies the Salaat).
For this reason, my Master, Hadhrat Ali Khawwaas (Rahmatullah alayh) said: ‘During the Six Fasts, it is necessary to be as diligent as in Ramadhaan. In fact, greater diligence should be observed during these Six Fasts because they are Jawaabir (i.e. compensate for the deficiencies; they eliminate the defects and rectify the Fasts of Ramadhaan which were executed defectively).’
If the Jawaabir are also executed deficiently, then the objective is not attained.” According to some Ulama, it is preferable to fast the Six Days consecutively. Fasting the Six Days altogether cultivates greater glitter in the heart.
It is narrated in Muslim, Abu Daawood, Tirmizi, Nisaa’, Ibn Maajaha, etc. that Rasulullah (Sallallahu alayhi wasallam) said: “Whoever fasts Ramadhaan and follows it with six days of Shawwaal, is like one who has fasted the whole year.”
This discourse adequately rebuffs the deviate Salafis who decry the Six Fasts of Shawwaal.