Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is nothing authentic that has been established that the Prophet (peace be upon him) held night vigil prayer and fasted during the day of the 15th of Sha`ban.
Responding to the question, Dr. Sano Koutoub Moustapha, Professor of jurisprudence and its principles at the International Islamic University, Malaysia, states:
It is true that there is no authentic hadith which calls upon Muslims to perform special prayers, fasting or special worship in the month of Sha`ban.
Having said this, I shall mention that there is an authentic hadith which states that the Prophet (peace and blessings be upon him) used to fast most of the month of Sha`ban and Usamah ibn Zaid asked him why he fasts most of this month. He replied by saying, “It is a month that the people forget about it, it is between Rajab and Ramadan, it is a month in which all deeds are brought to the attention of the Almighty. Therefore, I would like my deeds to be presented to Allah while I am fasting.” (Al-Bukhari)
According to this hadith, it is strongly recommended to fast in this month as much as we can, while bearing in mind that it is not allowed to fast the whole month. Furthermore, it is strongly recommended to keep imploring the Almighty to bestow upon you good health and enable you to fast the holy month of Ramadan.
Allah Almighty knows best.