All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There are several kinds of fasting. Among these kinds are:
1) Obligatory: observing the fast during the whole month of Ramadan is an obligation for all Muslims.
2) Supererogatory: this category includes fasting on particular occasions such as the 9th and 10th days of Muharram, the day of Arafah, fasting three days each month, and fasting on Mondays and Thursdays. It is a Sunnah to observe fasting on these days.
As for fasting on the day of al-Israa and al-Mi'raaj, it is not proved in the Sunnah of the Prophet . If it was desirable to fast on this day, the Prophet would have stated it as we have mentioned concerning supererogatory fasting.
Thus, it is not correct to observe fasting on this specific day.
Also, there is a difference of opinion among the scholars concerning singling out the day of al-Israa. Ibn Hajar in al-Fat'h mentioned that there are more than ten opinions in this concern. This disagreement indicates that this day has no special significance for fasting nor spending its night in performing prayers. If it was the case, the Prophet and his companions would not have missed it.
Based on the above-mentioned, we state that it is Bid'ah (innovation) to fast this day believing in its excellence since there is no correct Prophetic narration in this concern.
But, if fasting on this day coincides with any supererogatory fasting or one's regular fasts, such as fasting Mondays and Thursdays, or fating every other day, then it is permissible to fast this day (27th of Rajab).
As for spending the night of 15th of Sha'baan in prayers and other forms of worship, there are several Ahadeeth in this concern, but there is a difference of opinions among the scholars concerning the authenticity of these Ahadeeth. Some scholars consider them weak; other scholars consider them sound. Thus, according to some jurists, it is desirable to spend this night in optional forms of worship such as prayer …
According to Ibn Taymiyah, some righteous predecessors were used to spending this night (15th of Sha'baan) in prayers and other forms of worship.
But, some of them disliked gathering people at the mosque for this night and considered that as Bid'ah.
Nowadays, many Muslims perform a special prayer with special recital on this night. This form certainly is a Bid'ah.
Imam al-Nawawi said: “A prayer that is known as Salat al-Raghaeb, which is a prayer consisting of 12 Raka'hs, performed between al-Maghrib and al-Isha at the 1st Friday night of Rajab and a prayer on the 15th of Sha'baan (100 Raka'hs) are two forms of Bid'ah and are abominable actions. We should not observe them due to being mentioned in the books of al-Ihyaa and Qut al-Quloob. Then, their Hadeeth is false.”
The Hadeeth to which al-Nawawi referred to in the above paragraph is as follows: 'Whoever prays 100 Raka'hs on the night of 15th of Sha'baan, Allah will send one hundred angels for him. Thirty one give him glad tidings concerning Paradise; thirty others give him more confidence that he will be safe from entering Hell; another thirty keep him safe from the evils of this present life; the last ten ward the wiles of the devil off him'. This Hadeeth is not authentic, al-Nawawi said.
Finally, a Muslim is required to adhere to the obedience of Allah on this night (15th of Sha'baan) and on all other nights, to avoid all forms of polytheism and to pardon his brothers so that he may achieve goodness. Then, he is required to avoid all forms of Bid'ah, since good lies in following the Sunnah and evil lies in innovations.
Allah knows best.