This excerpt is taken from the book of Fadhilat al-Shaykh Mubashir Ahmad Rabbani, may Allah have mercy on him, Ahkam wa Masail Ramadan.
The Ruling on the Fasting of the 15th of Sha'ban:.
Fatwa:.
Fasting on these three days in every month is recommended, whether they are in Sha'ban or any other month. As it has been established from the Prophet (peace be upon him) that he instructed Abdullah bin Amr bin Aas (may Allah be pleased with him) regarding this.
Reference: (Bukhari, Book of Fasting, Chapter on Fasting of the Year: 1976, Muslim, Book of Fasting, Chapter on Prohibition of Fasting for a Long Time: 1159)
It is also established from the Prophet (peace be upon him) that he advised Abu Darda (may Allah be pleased with him) and Abu Huraira (may Allah be pleased with him) regarding this. And if a person fasts on these days in some months and skips in others, or sometimes fasts and sometimes skips, there is no issue. Because these are voluntary fasts, not obligatory, and it is better that if someone is able, they should continue to fast on these days every month.
Reference: (Muslim, Book of Prayer for Travelers, Chapter on the Recommendation of Duha Prayer: 722, Bukhari, Book of Fasting, Chapter on Fasting of the White Days: 1981)
