Beliefs

The birth of Al-Mahdi (as) had many similarities to those of the Prophet Moses. Prophet Moses's (as) birth had signaled the downfall and extinction of the empire of Pharaoh who had ordered the slaying of all the newly born children of Bani-Israel. The Abbasid caliphs were similarly apprehensive of the continuous narrations of the Holy Prophet (saws) about the birth of Al-Mahdi (as), who was to bring about a curse on their Abbasid Empire. Therefore they were lying in ambush to discover the birth of the Imam and to put an end to his life, but the event of the Imam's birth was enveloped and shielded by the same Divine protection and miraculous phenomenon that had marked the historical birth of Prophet Moses (as). His birth remained strictly confidential and his family were shrouded in secrecy except from a few devotees. The Imam's birth coincided with the reign of al-Mu'tamad, the well-known Abbasid caliph who being aware of the prophecy of the Twelfth Imam's birth occurring in his reign was exceptionally worried and anxious to find him.

The birth of Al-Mahdi (as) has been sufficiently proved in the Qur'an and ahadeeth. Eminent Sunni scholars such as Ibraheem Ibn Muhammad Hamwini Shaafi (in his book "Faraid-al-Simtain") and Abu Naeem Esfahani (in his book "Hilliyat-ul-Auliya"), have clearly discussed the birth of Al-Mahdi (as). The Prophet (saws) is reported to have said the following words about the Imams:

Hassan and Husayn will be, after their father (Ali), the Imams of my ummah and they will be the Masters of the youths of paradise, their mother is the leader of all Muslim women and their father is the Master of all the Muslim leaders. There will be born nine Imams from Husayn's progeny and all of them will be my great grandchildren, and whosoever loves and respects them loves and respects me, and one who offends them offends me as well.

Shaikh Najmuddin Askari - a Shi'ah scholar- in his book "Al-Mahdi al-Mao'od-ul-Muntadhar" has written two volumes on the Twelfth Imam. There are many other works such as Sheikh Saduq's "Ikmaal-ud-Din" and Ayatullah Safi Gulpayegani's "Muntakhab al-At'har."
There are several other Sunni scholars who have mentioned the Twelfth Imam with the fullest details, such as Sheikh Muhayy al-deen Ibn Arabi in his book "Al-Futuhat Al-Makiyya".
Ibn Muhammad Hamwini Shaafi writes in his Faraid-al-Simtain quoting from Dabill Khiza'ee that the latter went to Al-Ridha (the 8th Imam as) and read a eulogy in praise of Al-Mahdi (as), and said:

Al-Ridha (as) interrupted him and said: "O Dabill, you have uttered the words of fate, but do you know who that Imam would be?" Dabill replied that he would be from the Imam's progeny and will bring justice and fairness to a gloomy Earth. Al-Ridha (as) said that after himself, there would come his son Muhammad then his son Ali then his son Hassan and then his son Al-Mahdi (as), the last Imam for whom the entire world is waiting anxiously. He will eliminate oppression and injustice and fill the Earth with justice and peace.34
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34. Ibid: p.21

Adapted from the book: "The Awaited Saviour; Questions and Answers"