Hujjat Allah ibn al-Hasan al-Mahdi (Arabic: ?????? ???? ???? ???????? ????????????), also known as Imam al-Mahdi (Arabic: ????????? ????????????) and Imam al-Zaman (Arabic: ????????? ??????????) (meaning "Imam of Our Time"), Hujjat ibn Hasan and Muhammad ibn Hasan ibn Ali or Muhammad ibn Hasan, is believed by Shia Twelver Muslims to be the Mahdi, an eschatological redeemer of Islam and ultimate savior of humankind and the final Imam of the Twelve Imams who will emerge with Isa (Jesus Christ) in order to fulfill their mission of bringing peace and justice to the world.
Twelver Shias believe that al-Mahdi was born on the 15th Sha'ban 879 CE/ 255AH and assumed Imamate at nearly 5 years of age following the killing of his father Hasan al-Askari.
In the early years of his Imamate, he is believed to have had contact with his followers only through The Four Deputies.
After a 69-year period, known as the Minor Occultation (Arabic: ?????????? ????????????), a few days before the death of his fourth deputy Ali ibn Muhammad al-Samarri in 948, he is believed to have sent his followers a letter.
In that letter which was transmitted by al-Samarri he declared the beginning of Major Occultation (Arabic: ?????????? ????????????), during which Mahdi is not in contact with his followers directly but has instructed them to follow the pious high clerics for whom he has mentioned some distinguishing merits.
Sunni Muslims reject that he is the Mahdi and believe that the Mahdi has not yet been born.
They believe his exact identity is only known to Allah, other than the idea that he is to be from the descendants of Muhammad.
Aside from the Mahdi's precise genealogy, Sunnis accept many of the same hadiths which Shias accept about the predictions regarding the Mahdi's emergence, his acts, and his universal Caliphate.
Sunnis also have many hadiths about Mahdi which are in their Hadith collections.