The Mahdi from among the Descendants of the Prophet
Such hadith-reports are abundant. The main idea that runs through all of them suggests that the topic about the future coming of the Mahdi and Qa’im during the time of the Prophet (peace be upon him and his progeny) was well known. In fact the way these reports speak about the subject indicates that it was not something new which was being presented to the people. On the contrary they relate the signs and characteristics of the person who would emerge as the Mahdi as in the statement "the promised Mahdi will be among my descendants."
The following traditions reflect such a pattern in their presentation. It is reported from Ali bin Abi Talib who said:
I asked the Prophet: "Is Mahdi going to be among our own family or from some other?" He replied: "He will be among us. God will conclude His religion through him just as He began it with us. It will be through us that people will find refuge from sedition just as it was through us that they were saved from polytheism. Moreover it will be through us that God will bring their hearts together in brotherhood following the animosity sown by the sedition just as they were brought together in brotherhood in their religion after the animosity sown by polytheism."
Abu Sa’id al-Khudari a close associate of the Prophet says:
"I heard the Prophet declare from the pulpit: "The Mahdi from among my descendants from my family will rise at the End of Time while the heavens will pour rain and the earth will bring forth green grass for him. He will fill the earth with justice and equity as it is filled with tyranny and injustice."
In another tradition from Umm Salma the wife of the Prophet there is even more specific information given to the community. The Prophet says: "Mahdi will be among my progeny among the children of Fatimah."
On another occasion the Prophet said:
"The Qa’im will be among my descendants. His name will be my name and his patronymic will be my patronymic. His character will be like my own. He will call people to my custom and to the Book of God. Anyone who obeys him would be obeying me and any one who turns away from him would be turning away from me. Anyone who denies his existence during his concealment would have denied me and anyone who falsifies him would have falsified me. Anyone who confirms his existence would have confirmed my existence. As for those who are engaged in falsifying what I have said about him and thereby mislead my community I will complain against them to God. "Those who do wrong shall surely know by what overturning they will be overturned." ( 28:88) 
Abu Ayyub Ansari says: I heard the Prophet (peace be upon him and his progeny) say: "I am the chief of the prophets and Ali is the chief of the legatees. My two grandsons are the best among the descendants. The infallible Imams will come forth from among us through Hussein. Moreover the Mahdi of this community is among us." At that time an Arab stood up and asked: "O Prophet of God how many Imams are there after you?" He replied: "Equal to the number of the apostles of Jesus and the chiefs of the Children of Israel." 
A tradition with similar information has been cited from Hudhayfa another companion of the Prophet who heard the Prophet declare:
The Imams after me will be equal to the number of the tribal chiefs among the Children of Israel. Nine among them will be the descendants of Hussein. The Mahdi of this community is among us. Beware! Truth is with them and they are with truth. Thus be careful of the way you treat them after me. 
In still another tradition Sa’id b. Musayyib reports from Amr b. Uthman b. Affan who said: We heard from the Prophet saying:
"The Imams after me will be twelve in number of whom nine will be from the progeny of Hussein. Moreover the Mahdi of this community will be among us. Anyone who holds on to them after me holds on to the rope of God; and whoever abandons them has abandoned God." 
There are numerous hadith-reports of this kind in the sources which one can undertake to examine.
Note: This time we will work on just ten parts of this useful book but as always accompany us for other parts of this book in future.
(6). Ibid. Vol. 51 p. 84; Ithbat al-hudat Vol. 7 p. 191; Majma" al-zawa’id by "Ali b. Abi Bakr Haythami (Cairo edition) Vol. 7 p. 317.
(7). Bihar al-anwar Vol. 51 p. 74; Ithbat al-hudat Vol. 7 p. 9.
(8). Bihar al-anwar Vol. 51 p. 75.
(9). Bihar al-anwar Vol. 51 p. 73.
(10). Ithbat al-hudat Vol. 2 p. 531.
(11). Ibid. p. 533.
(12). Ibid. p. 526.
Source: The book named The Just Leader of Humanity
By Ayatullah Ibrahim AminiTranslated by: Dr. Abdulaziz Sachedina
Imam Al-Mahdi Al-Muntadhar (A.S): part 1
Imam Al-Mahdi Al-Muntadhar (A.S): part 2
Imam Al-Mahdi Al-Muntadhar (A.S): part 3
Imam Al-Mahdi Al-Muntadhar (A.S): part 4
The sudden nature of the advent of al-Qa’im(A.S)