I know it is Sunnah to fast three days every month and that Nabi (sallallahu ‘alayhi wa sallam) would fast on the 13th, 14th and 15th of every month as well as every Monday and Thursday.
My question is which method from the aforementioned combinations will be Sunnah?
Fasting on Monday and Thursday is indeed recommended as Nabi (sallallahu ‘alayhi wa sallam) would observe these fasts.
(Sahih Ibn Khuzaymah, Hadith: 2116, 2119, 2120. Also see Sahih Muslim, Hadith: 1162)
There are many narrations wherein Nabi (sallallahu ‘alayhi wa sallam) encouraged to fast three days a month and some authentic narrations specify these days to be the 13th, 14th and 15th of every month.
Hafiz ibn Hajar (rahimahullah) quotes Imam Ruyani who says, It is Mustahab (desirable) to fast three days a month and it is more desirable to keep these three fast on ‘Ayyamul bid’ [the 13th, 14th and 15th].
(Refer: Fathul Bari, under Hadith: 1981)
Both these combinations are desirable, A person should strive to fast on Mondays and Thursdays as well as the 13th, 14th and 15th of every month. If this is not possible, then a person should at least fast the ‘Ayyamul Bid’ as Rasulullah (sallallahu ‘alayhi wa sallam) said:
“Fasting three days a month, the Ayyamul Bid from the 13th, is equivalent to fasting your entire life”
(Sunan Nasai with a reliable chain. Refer: Fathul Bari, Hadith: 1981)
See here for an article regarding the Mustahab fasts of the year.
And Allah Ta’ala Knows best.
Answered by: Moulana Suhail Motala
Approved by: Moulana Muhammad Abasoomar
Checked by: Moulana Haroon Abasoomar