** more on subject**
1. Sunni reference: Musnad Ahmad Ibn Hanbal, v1, p99
Ibn Majah in his Sunan quotes Mohammad Ibn Hanafiyyah and Imam Ali saying that the Holy Prophet (PBUH) said:
"Mahdi is from our Ahlul-Bayt, no doubt Allah will enforce his Amr
(appearance) within a night (i.e., his coming is very unpredicted)."
Sunni reference: Sunan Ibn Majah, V2, P269
The more recent fatwa in this issue is given in Mecca by the Muslim World League (Rabitatul `Alamul Islami) on Oct. 11, 1976 (23 Shawwal 1396).
This fatwa states that more than twenty companions narrated traditions concerning al-Mahdi, and gives a list of those scholars of Hadith who have
transmitted these narrations, and those who have written books on al-Mahdi. The fatwa states:
"The memorizers (Hafiz) and scholars of Hadith have verified that
there are authentic (sahih) and acceptable (hasan) reports among the
traditions related to al-Mahdi. The majority of these traditions are
related through numerous authorities (Mutawatir). There is no doubt
that the status of those reports are Sahih and Mutawatir. (They have
also verified) that the belief in Mahdi is obligatory, and that it is
one of the beliefs of Ahl al-Sunnah wal Jama'a. Only those ignorant of
the Sunnah and innovators in doctrine deny it.
For the transcription and reproduction of this fatwa, see, among others, the Introduction of al-Ganji al-Shafi'i, in the book named "al-Bayan," Beitut,
1399/1979, PP 76-79 and in Appendix.
Among Shi'ite scholars, I would like to mention the masterpiece of Lutifullah Al-Safi al-Golpaygani, who compiled an encyclopedia called "Muntakhab
al-Athar". In this book, there is a comprehensive narration of the traditions relating to the appearance of the Mahdi (AS). He has used over 60 Sunni
sources, including their Six Authentic Books of Tradition; and over 90 Shi'ite sources to illuminate the fact that Al-Mahdi is NOT a fabricated event.
The Prophet (PBUH&HF) said: "He who dies not knowing the Imam of his era, has died the death of Al-Jahiliyya (Days of Ignorance; the era before
Islam)."