Shaykh Uthaymeen said, “That which is correct is that fasting the 15th of Sha’baan or specifying it with reciting (the Quran) or making (particular) supplications has no basis.
So the day of the 15th of Sha’baan is like any other 15th day of other months.
So from that which is known is that it has been legislated for a person to fast the 13th, 14th and 15th of every month, however, Sha’baan is characterised unlike the other months in that (except for Ramadhaan) the Prophet (sal-Allaahu `alayhe wa sallam) used to fast more in Sha’baan than any other month.
So he used to either fast all of Sha’baan or just a little.
Therefore, as long as it does not cause difficulty for a person, it is befitting to increase in fasting during Sha’baan in adherence to the example of the Prophet (sal-Allaahu `alayhe wa sallam).”
Ref: al-Bid’u wal-Muhdathaat wa maa laa Asla lahu – Page 612 Fataawa Shaykh Muhammad Ibn Saalih al-’Uthaymeen – Volume 1, Page 190