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. (Fadhaail Al 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 ‘Ummaal, Hadith No. 35169; Maa Thabata Bis Sunnah, Pg. No. 70; Al Ghunya Li Talibee Tareeqil Haqq, Vol. 1, Pg. No. 182/183, Tafseer Durr Al Manthoor, Surah Tauba-36)
The aforementioned Hadith make it clear that engaging in worship on the 27th night of Rajab is the Sunnah of the Holy Prophet (Sallallahu alaihi wa sallam) and the Sunnah of the Companions. Fasting on the 27th day has 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.