The Messenger of Allah, Muhammad (peace and blessings of Allah be upon him), said:
“Whoever fasts during the month of Ramadan and follows it with six days of fasting in the month of Shawwal, it will be as if he had fasted for the entire year.”
— Sahih Muslim
The Advice of the Prophet (ﷺ) to His Companions
Narrated by the Companion Abu Hurairah (may Allah be pleased with him):
“My beloved friend (the Prophet, peace and blessings of Allah be upon him) advised me to do three things: to fast three days of every month, to pray the two rak’ahs of Duha prayer, and to pray the Witr prayer before going to sleep.”
— Sahih Al-Bukhari
Fasting three days in every month is Mustahabb (highly recommended).
Narrated by Abu Darda (may Allah be pleased with him):
“My beloved (the Prophet, peace and blessings of Allah be upon him) advised me to do three things which I will never abandon until I die: to fast three days of every month, to pray the Duha prayer, and not to sleep until I have prayed the Witr prayer.”
— Sahih Muslim
