Imam al-Mehdi (AS) is the hope not only for Muslims but for all oppressed human beings who suffer slavery, oppression, and subjection. He is the Savior who will free the will of man and save peoples and nations from the oppression of the corruptive regimes who have turned this world into unbearable hell.
Imam al-Mehdi (AS) was and is a great miracle in Islam. Allah concealed his birth, as he had concealed the birth of Prophet Moses (AS), because the Abbasid government looked for him…and in his remaining alive (in occultation) throughout the long stages of life is also a miracle for Islam, and in his reappearance and announcing the pure principles of Islam is another miracle too.
The following are some of the traditions transmitted from Imam Abu Muhammad al-Hasan al-Askari (AS) concerning the Imamate of his son al-Mehdi (AS).
1. Ahmad bin Isaaq bin Sa’eed al-Ash’ari said, “Once, I went to Abu Muhammad al-Hasan bin Ali (AS) intending to ask him about the successor after him. He said to me before I asked him, ‘O Ahmad bin Isaaq, Allah the Almighty did not deprive the earth since He created Adam and He does not deprive it until the Day of Resurrection of an authority over His people. By him (the authority) distresses are pushed away from the people of the earth, by him rain falls down, and by him the blessings of the earth are emitted.’
I said to him, ‘O son of the messenger of Allah, who is the imam and successor after you?’ He got up hastily and went in the house. Then, he came out holding a three-year-old boy, whose face was like a full moon, on his shoulder and said, ‘O Ahmad, were it not for your honor near Allah the Almighty and near His authorities, I would not show you my this son. He was named and surnamed like the messenger of Allah (AS).
He will fill the earth with justice and fairness after it has been filled with injustice and oppression. O Ahmad, he, in this nation, is like al-Khidhr (Elijah) and Thul Qarnayn. By Allah, he will hide in an occultation, during which no one shall be saved except one whom Allah fixes on the believing in his imamate and supplicating Allah to hasten his deliverance.’ I said, ‘Is there any sign so that I may be sure?’ The young boy said, ‘I am the representative of Allah in His earth and the avenger on His enemies. Do not look for a sign after a proof!’
I left delightedly. On the following day, I came back to him and said, ‘O son of the messenger of Allah, my delight was so great for what you favored me with. Would you please tell me what norm with al-Khidr and Thul Qarnayn was?’
He said, ‘Long occultation.’
I said, ‘O son of the messenger of Allah, shall his (Imam al-Mehdi’s) occultation last long?’
Imam Abu Muhammad (AS) said, ‘Yes, by my Lord, until many of those, who believe in this matter (occultation and reappearance), shall apostatize, and no one shall remain (believing) except those whom Allah has taken from them a covenant on our guardianship, fixed faith in their hearts, and supported them with a mercy from Him. O Ahmad, this is a command from Allah, a secret from the secrets of Allah, and an unseen matter from the unseen of Allah. Take what I told you, and be from the grateful, you shall be with us in Illiyin.”
 Illiyin is the highest position in the Paradise.
 Ikmaluddeen by Sheikh as-Saduq, p.216-217.
2. Muhammad bin Uthman al-Umari narrated that his father said, “Once, I was with Abu Muhammad al-Hasan bin Ali (al-Askari) when he was asked about the tradition transmitted from his father that the earth would not be empty from an authority from Allah over His people until the Day of Resurrection, and that whoever died without knowing the imam of his age would die as an unbeliever. He said, ‘This is true as day is true.’
Someone asked him, ‘O son of the messenger of Allah, then, who shall be the authority and imam after you?’
He said, ‘My son Muhammad is the imam and the authority after me. Whoever dies without knowing (acknowledging) him shall dies as an unbeliever. He will be in an occultation where the ignorant will be confused, deniers will perish, and daters will tell lies. Then, he will reappear, and as if I see white flags fluttering over his head at the hill of Kufa.’”
3. Imam Abu Muhammad (AS) said, ‘Praise be to Allah Who did not take me out of this life until He made me see the successor after me. He is the most similar in shape and morals to the messenger of Allah (AS). Allah will protect him in his occultation and make him reappear to fill the earth with justice and fairness after it will be filled with injustice and oppression.’
4. Musa bin Ja’far al-Baghdadi narrated that he had heard Imam Abu Muhammad al-Hasan bin Ali al-Askari (AS) saying, ‘As if I see that you will disagree after me on my successor.
 Those who defines certain dates for the reappearance of the awaited imam (AS).
 Kifayat al-Athar.
 Kifayat al-Athar, Ikmaluddeen, p.228.
Surely, he, who believes in the imams after the messenger of Allah (AS) but denies my son, is like one, who believes in all prophets and messengers of Allah but denies the prophethood of the messenger of Allah (Muhammad) (AS), because the obedience of the last of us is like the obedience of the first of us, and one, who denies the last of us, is like one who denies the first of us. My son will have an occultation where people will be in doubt about it except those whom Allah preserves.’  Kifayat al-Athar.
Taken from : The Life Of Imam Al-Hasan Al-Askari Study And Analysis By: Baqir Shareef al-Qurashi Translation by: Abdullah al-Shahin