Worship on the 27th night of Rajab (the night after 26th) is well established by the Hadith.
There is a Hadith:
Translation: Hadhrat Anas bin Malik (May Allah be well pleased with him) relates from the Holy Prophet (Sallallahu alaihi wa sallam) that He (Sallallahu alaihi wa sallam) said: There is such a night in the month of Rajab in which virtues worth 100 years are written for those who act and that is the 27th night of Rajab. The person who offers 12 Raka’at of Salaat this night in such a way that he (or she) recites any Surah after Surah Fatiha, in units of 2 Raka’at each and finishes the Salaat and after finishing the Salaat, recites Subhan Allah 100 times, performs “Istighfaar” 100 times and sends Durood on the Holy Prophet (Sallallahu alaihi wa sallam) 100 times and asks for whatever good he (or she) wants of this world or the hereafter and fasts the next day, Almighty Allah will certainly accept all his supplications except that he makes supplicates for some disobedience (that supplication will not be accepted). (Fadhaailil Auqaat Lil Baihaqui, Hadith No. 12; Shua’bul Iman, Hadith No. 3651; Jame’ Ul Ahadeeth, Hadith No. 41812; Jame’ Ul Kabeer, Hadith No. 171; Kanz Ul ‘Ummal, Hadith No. 35170. Ma Thabat As Sunnah, Pg. No. 70; Al Ghunya Lit Talibi Lit Tareeqil Haqq, Vol. 1, Pg. No. 182/183; Tafseer Durre Manthoor, Surah Tauba-36)
Excellence of the 27th day and its worship
Hadhrat Ghouse Azam (May Allah be well pleased with him) has related a Hadith with his own chain of authority about the excellence of fasting on the 27th day of Rajab:
Translation: Hadhrat Abu Hurairah (May Allah be well pleased with him) said: The Holy Prophet (Sallallahu alaihi wa sallam) said: For the person who fasts on the 27th of Rajab, the reward of fasting for 60 months is recorded. (Al Ghunya Lit Talibi Lit Tareeqil Haqq, Vol. 1, Pg. No. 182)
There is another Hadith in the same book:
Translation: It is narrated on the authority of Hadhrat Hasan Basri (May Allah shower His mercy on him), he says: Hadhrat ‘Abdullah bin ‘Abbas (May Allah be well pleased with them) would put himself in seclusion (‘Itikaaf) on the 27th of Rajab. He would keep offering Salaat until Salaat Ul Zuhr. When he would offer Salaat Ul Zuhr, he would move a bit to one side and offer 4 Raka’at. In each Raka’at, he would recite Surah Fatiha and Surah Falaq and Surah Naas once each and Surah Qadr thrice and Surah Ikhlaas 50 times. Then he would supplicate until Salaat Ul ‘Asr and would say: This would be the custom of the Holy Prophet (Sallallahu alaihi wa sallam) on this day. (Al Ghunya Lit Talibi Lit Tareeqil Haqq, Vol. 1, Pg. No. 182)
The excellence of worship in Shab Meraj and of fasting on the 27th of Rajab
Imam Abu Bakr Ahmed bin Hussain Baihaqui, Imam Suyuti, ‘Allama ‘Ali Muttaqi Al Hindi and Hadhrat Shah ‘Abdul Haq Muhaddith Dehlvi (May Allah shower His mercy on them) have reported a Hadith in their respective books:
Translation: It is narrated on the authority of Hadhrat Salman Farsi (May Allah be well pleased with him), he says that the Holy Prophet (Sallallahu alaihi wa sallam) said: There is such a day in the month of Rajab and such a night that whoever fasts on that day and stands up in worship in that night has as if fasted for 100 years and spent 100 years in worship and that is the 27th night of Rajab. (Fadhaailil Auqaat, Hadith No. 11; Shua’bul Iman, Hadith No. 3650; Jame’ Ul Ahadeeth, Hadith No. 14813; Al Jame’ Ul Kabeer, Hadith No. 172; Kanz Ul ‘Ummal, Hadith No. 35169; Maa Thabat As Sunnah, Pg. No. 70; Al Ghunya Lit Talibi Lit Tareeqil Haqq, Vol. 1, Pg. No. 182/183, Tafseer Durr Al Manthoor, Surah Tauba-36)
The aforementioned Hadith make it clear that spending Shab Meraj in worship is the Sunnah of the Holy Prophet (Sallallahu alaihi wa sallam) and the Sunnah of the Companions. Fasting on the 27th of day leads to much excellence and reward. Stopping people from these forms of worship and declaring it as a reprehensible innovation is not at all allowed by the Shariah.