Reports of the birth of the Bab have been reported by Bahai historians only.
The reports of the birth of the Mahdi of Islam have been reported by both, Shia as well as Sunni historians.
As the column alongside shows, the birth of the Mahdi had already taken place more than 1,000 years prior to the birth of the Bab. Then where is the question of the Bab being the promised Mahdi of Islam?
The Mahdi of Islam, on his birth, went into prostration and recited the verse, "Surah Bani Israel, verse 81".
وَقُلْ
جَاء
الْحَقُّ
وَزَهَقَ
الْبَاطِلُ
إِنَّ
الْبَاطِلَ
كَانَ
زَهُوقًا
And say: "Truth has (now) arrived, and Falsehood perished: for Falsehood is (by its nature) bound to perish."
His father Imam Hasan Askari (as) even introduced his son, the Mahdi to his trusted companions and informed them of his successor. The father of Mirza Ali Mohammed, The Bab did neither. Perhaps he was aware that his son is a "Mirza" and not a "Sayyed".
The Bab himself testifies in his book, Tafseere' Surah Kauthar, that he saw a person standing near the Holy Ka'bah who the Bab considered as the Mahdi and wanted to approach him. When the Bab himself considered another person to be the Mahdi, then why are the Bahais claiming otherwise?