Q: What is the ruling of fasting the entire month of Sha’ban?
A: There are numerous ḥadīth that demonstrate the virtue of abundant fasting in the month of Sha’ban.
Fasting the month, in its entirety, is not disliked as it is for the month of Rajab because the Prophet (peace be upon him) would fast Sha’ban and Ramadan. What is understood from this practice, however, is that it was done only occasionally and so the majority do not deem it recommended to fast the month in its entirety.
Source: Sharh al-Muntaha, Kashshaf al-Qina’, al-Insaf