Muhammad ibn Hasan al-Mahdī (Arabic: محمد بن الحسن المهدي) is believed by Twelver Shī‘a Muslims to be the Mahdī, an ultimate savior of humankind and the final Imām 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 Shī‘a believe that al-Mahdī was born in 869 (15 Sha‘bān 255 AH) and assumed Imamate at 5 years of age following the death of his father Hasan al-Askari. In the early years of his Imamate he would only contact his followers through The Four Deputies. After a 72-year period, known as Minor Occultation, a few days before the death of his fourth deputy Abul Hasan Ali ibn Muhammad al-Samarri in 941, he is believed to have sent his followers a letter. In that letter that was transmitted by al-Samarri he declared the beginning of Major Occultation during which Mahdi is not in contact with his followers.