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 Li Talibee 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 Li Talibee Tareeqil Haqq, Vol. 1, Pg. No. 182)
Abul Hasanaat Islamic Research Center 