"My successors after Me are God's proofs for the people of the earth. They are twelve. First one of them my brother and last of them will be my child. They asked: O Apostle of God who is your brother? He said Ali-ibn Abu Talib. They asked who is your child? He said: He is Mahdi who will fill the earth with justice, even if it is full of injustice and wretchedness. I swear to God Who made Me His Messenger even if one day is left from the age of the world, God will lenghten that day to the extent that My child Mahdi will become manifest. Then Issa [Maseeh], the Spirit of God descends (the Return of Christ] from heaven and pray behind Him."
According to Sunni sources: Allamih Jalal'ud'Din Suyooti in his book "Al Araf'ul Vardi fi Akhbaar al-Mahdi" offers a Hadith narrated by Ibn Abbas:
"The Messenger of God said: Al Mahdi is from My Household [He will arise from Muhammad's family line]. He will arise as the Vicegerent of God [Kalifa-tullah] and afterward He will relegate His Rulership to Isaa bin Maryam."
The two Ahadith quoted above are in total conformity with the Sahih Ahadith from Bukhari I have already mentioned. Their relevance to one another is indisputable. In order to further demonstrate the validity of the Sahih Ahadith I have quoted referring to the Twin appearances of the Lord as Mahdi and Maseeh, in the day of Resurrection, I'd like to present the following verses of the Qur'an:
"What can such expect but that God should come down to them overshadowed with clouds..." -Qur'an 2:210
The descent of God into the realm of creation is foretold in the above verse of the Qur'an. Please note that Allah is, according to the Ahadith I have cited in the above passages is to appear TWICE (1- the appearance of Mahdi and 2- the descent of Issa bin Maryam) and will speak to the ummah of Islam.
This expectation of the DOUBLE appearance of Allah is also affirmed in other passages in the Holy Qur'an:
"And there shall be a blast on the Trumpet, and all who are in the Heaven and all who are on the earth shall expire, save those whom God shall vouchsafe to live. Then shall there be another blast on it, and lo! arising they shall gaze around them: and the earth shall shine with the light of her Lord." -Qur'an 39:68-69 [Note: The title Baha'u'llah can be translated as 'the Light of the Lord']
Please note how clearly the text of the Qur'an foretells the appearance of two successive Resurrections, or the Twin appearances of the Lord. Moreover, please note how clearly the Text of the Qur'an implies the meaning of the title of Baha'u'llah (Light of God, Splendor of God, Glory of God)
"One day, the disturbing trumpet-blast shall disturb it [the universe] which the second blast shall follow.... Verily, it will be but a single blast..." -Qur'an 79:6,7&13
According to this passage the Revelation of Mahdi and Maseeh will be inseparable from one another, as actually was the case with the Revelation of The Bab and Baha'u'llah.