Leave aside Shiah historians, even authors from Sunni sects have recorded this event in their works. Given below are some of the historians who documented the birth of Imam Mahdi. For some, I have brought forth not only the names of the books and the authors, but also their statement. For others I have brought their names and their books. If any reader wishes to see the actual statements of these authors, I will be happy to provide it on request.
Ibne Abi Salij Baghdadi (d. 326 AH) a contemporary of the deputies of Imam Mahdi (as) in his book Mawaleedo Wafaatul Aimmah, while narrating the events of 260 AH writes, "At the time of his death, Imam Hasan Askari (as) was survived by this children Mohammed, Musa, Fatemah and Ayesha." The author also states that at the time of the birth of Mohammed Ibnil Hasan Askari, the Imam remarked, "The enemies wanted to kill me while I was not having any son. But a son has been born to me. He will be the desire of the people."
Ali bin Hussain Mas'oodi (d. 346 AH) in his book Moruj uz Zahab (volume 4, page 199) while narrating the events of 260 AH writes that Abu Mohammed Hasan ibne Ali was martyred at the age of 29 during the reign of Mo'tamid. He was the father of the 12th Imam of the Shiahs, Imam Mohammed Al Muntazar.
Abu Bakr Khwarazmi (d. 383 AH) on page 32 of his book Mafatihul Uloom under the chapter of "Characteristics of the Leaders of the Imamiyahs" writes that Mohammed Mahdi Al Qaem Al Muntazar is alive. He will remain alive till such time that he does not fill the earth with justice and equality.
Hafiz Abu Bakr Ahmed Ibne Hussain Bayhaqi Shafa'ei (d. 458 AH) writes in his book Sho'bal Eeman, Imam Mahdi was born on 15th Shabaan 255 AH. He is alive till today and in occultation. The Shiahs are anxiously awaiting his re-appearance for he will, on his re-appearance, fill the earth with justice and equality.
Shaykh Ahmed Jami (d. 536 AH). Shaykh Sulaiman Qunduzi Hanafi has narrated poetic verses from him in his book Yanabiul Mawaddah which indicate that he testified to the birth of Mohammed Ibnil Hasan Sahebaz Zaman.
Yahya bin Khaskafi Shafa'ei (d. 553 AH). Sibt Ibne Jauzi in his book Tazkeratul Khawasul Aimmah, page 360 brings forth poetic verses to the effect that Imam Ali Naqi (as) was succeeded by Imam Hasan Askari (as) and after him, his son Imam Mohammed Ibnil Hasan (as) who survived him.
Ibne Khashab Baghdadi (d. 567 AH) writes in his book Taarikhe' Mawaleedul Aimmah wa Wafaatehim, "Khalafus Saleh is amongst the children of Imam Hasan Askari (as). He alone is the Sahebaz Zaman and the Mahdi."
An Nasir Le Deenillah, the Abbasi Caliph, Ahmed bin Mustazeeyi in the year 606 AH ordered restoration work to be carried out in the Sardab (cellar) - the house of Imam Hasan Askari (as) and the place from where his son went into occultation.
Yaqut Ham'oui (d. 626 AH) writes in his book Mo'jamul Baladaan, Qaeme' Mohammed is stationed at the Sardab. (The Shiahs believe that Imam Mahdi (as) does not reside there. He (as) however does visit the Sardab from time to time).
Shaykh Fida'ud Deen Attar (killed in 677 AH) while narrating the history of all the Imams (as) in Mazharas Sifaat quotes a few verses of poetry in praise of Imam Mahdi (as) to the efect, O Allah! Hasten the re-appearance of the Mahdi so that he (as) may fill the earth with justice and equality.
Ibne Aseer Juzri (d. 630 AH) while narrating the events of the year 260 AH in his book Taarikhe' Kamil writes, Hasan Ibne Ali, the father of Mohammed passed away from this world…
Shaykh Sa'ad ud Deen Ham'oui (d. 650 AH) has narrated the conditions and characteristics of Imam Mahdi (as) in a significant work in which he narrates, the last Imam is the Qaem and the Mahdi.
Ibne Talha Shafa'ei has compiled his book Matalebus So'aal Fi Manaqebe' Aale' Rasul in 12 chapters. The twelfth chapter is dedicated exclusively to Al Qaem Mohammed Ibnil Hasan….Al Mahdi Al Hujjat Al Khalafus Saleh Al Muntazar. In this chapter the compiler has in great detail described the characteristics of the last Imam on the same lines as the Shiahs.
Hafiz Mohammed Ibne Yusuf Shafa'ei (d. 658 AH) Ibne Sabbake' Maleki has addressed this historian as "Al Hafez" in his book Fosulul Mohimmah. His significant work Al Bayan Fi Akhbare' Sahebaz Zaman mentions clearly that the birth of Imam Mahdi (as) took place on 15th Shabaan 255 AH.
Shaykh Sadrudeene' Qunuwi (d. 673 AH) in a will to his students desired that on the first night after burying him, they should recite La ilaha illallah (There is no God but Allah) 70,00 times on his behalf and gift it reward to Imam Mahdi (as).
Ibne Khallakaan Ash'ari Shafa'ei (d. 681 AH) famous as Qaziul Quzaat in volume 3, page 316 of his book Wafayatul Aa'yaan writes Abul Qasim Mohammed bin Hasan Al Askari is the 12th Imam and Mahdi. His titles include Hujjat, Muntazar and Qaem. He was born on 15 Shabaan 255 AH and was 5 years old at the time of his father's martyrdom.
Abul Fida Emadud Deen (d. 732 AH) has narrated the incident of the martyrdom of Imam Hasan Askari (as) in volume 2, page 45 of his book Al Mukhtasar Fi Tarikhe' Bashar. He writes that Imam Hasan Askari (as) was the father of Mohammed who was born in 255 AH.
Hafez Shamsudeen Mohammed Zahabi (d. 748 AH) on page 115 of the first volume of his book Dulul Islam, that Imam Hasan Askari (as), the father of Mohammed Muntazar passed away in the year 260 AH.
Ibnul Waradi Zaynud Deen Umar Ibne Munfater (d. 749 AH) - the son of Abdur Rahman Ibne Abi Bakr (Jurist and Qazi of the city of Halab) - in his historical compilation Tarikhe' Ibnul Waradi, volume 1, page 318 published from Najaf with the preface of Sayyed Mohammed Aale' Khorasan clearly writes that Mohammed Muntazar was born in the year 255 AH.
Shaykh Abdullah Mateeri Madani Shafa'ei (d. 745 AH) has brought 51 traditions in praise of the Ahle Bayt in his book Riyazuz Zaherah Fi Fazlul Aale Baytin Nabi wa Itratehit Taherah. He acknowledges that the first Imam is Ali Ibne Abi Talib (as) and the 11th Imam, Imam Hasan Askari (as) and after him the twelfth Imam is his son Mohammed Al Qaem Al Mahdi (as). He will have 2 occultations following which he will arise with the sword.
Shaykh Abdullah Ya'fa'ei (d. 768 AH) in volume 1, page 172 of his book Meratul Jinaan documents the events of 260 AH in which Imam Hasan Askari, the father of Mohammed Muntazar was martyred.
Fazeeh Ahmed Khawafi (b. 777 AH) describes the events of 255 AH in his book Majmul Faseeh (volume 1, page 231) as follows - Al Mahdi (as) was born on 15th Shabaan 255 AH. He is Abul Qasim Mohammed bin Askari….he is the twelfth and the last Imam.
Ahmed Ibne Ali Dawoodi Hasani (d. 828 AH) in his book Umdatut Taleb published from Beirut. On page 163 while describing the conditions of Imam Hasan Askari (as) he writes, He (Imam Hasan) made a mark for himself in the aspects of peity and struggle. He was the faither of Mohammed Mahdi, the 12th Imam of the Shiahs. His (Imam Mahdi's) mother's name was Narjis.