Islam Guidance

Imam Hassan al-Askari (A.S) has said: "There are two characteristics, which surpass all others: having faith in Allah, and benefiting one's brothers (in faith)." (Tuhaf al-Uqul, p. 489)
One of the major responsibilities of Imam Hassan al-Askari (A.S), for which he endeavored earnestly, was to introduce the Imam after him to the Shiites and to inform them of the next Imam's occultation. At the time, the Abbasid dynasty, having had heard the words of the previous Imams (A.S) and those of Prophets'  (A.S) about the savior that would be born to Imam Hassan al-Askari (A.S), were in close watch of Imam Hassan's (A.S) residency to make sure he would not have a son. Thereupon, Imam Hassan al-Askari (A.S) had to hide his son from the guards sent by the government and in that manner save his son's life. After the martyrdom of Imam Hassan al-Askari (A.S), Imam Mahdi (A.S) went into the periods of his short and long occultation, whereupon most Shiites were deprived from seeing their Imam. Upon this matter, at the time of Imam al-Askari (A.S) it was necessary to introduce Imam Mahdi (A.S) to certain Shiites, and clarify his processes of occultation. Many narrations have been recorded on this matter, but in this excerpt we will refer to only one.
Ahmad ibn Is'haq, one of the reliable companions of Imam Hassan al-Askari (A.S), has narrated: "In one of my visits to the Imam, I had intended to ask about his successor. Yet before I had spoken, the Imam (A.S) said: 'O Ahmad ibn Is'haq! From the creation of Adam, Allah (S.W.T) has never left the earth without His successor on people, and will never do so until the Day of Judgment…'
I asked: 'O son of the messenger of Allah! So who is the successor and Imam after you?'
Imam Askari (A.S) stood up immediately, and entered the house. He then returned carrying a three year old boy, whose face was filled with a halo of light. He said:
'O Ahmad! If it were not for your eminence in the eyes of Almighty Allah and His chosen people, I would not have shown you my child. His name and nickname are the same as that of Prophet Muhammad's (P.B.U.H&H.P); and it is he who will fill the earth with justice after it has been corrupted with oppression…
By Allah (S.W.T)! He will live through a long occultation from which people will not survive perdition and deviation, except for those who stand firm on believing in his Imamate, and those who are honored with the opportunity to pray for haste in his reappearance.
Ahmad ibn Ishaq narrates: "the minute I asked Imam Askari (A.S): 'Will any sign or miracle appear to make me confident?' the child began speaking:
'I am the remainder of Allah on earth; the one who will take revenge on the enemies of Allah; and after seeing me, do not search for any other signs!"…

Derived from: Roshd Islamic Shia Website

Holy Prophet (P.B.U.H&H.P) in the sermon on the day of Ghadeer made mention of Imam Mahdi (A.S).

ألا إن خاتم الائمة منا القائم المهدي، ألا إنه الظاهر على الدين، ألا إنه المنتقم من الظالمين، ألا إنه فاتح الحصون وهادمها، ألا إنه قاتل كل قبيلة من أهل الشرك، ألا إنه المدرك بكل ثار لاولياء الله عزوجل، ألا أنه الناصر لدين الله، ألا إنه الغراف من بحر عميق، ألا إنه قسيم كل ذي فضل بفضله وكل ذي جهل بجهله، ألا إنه خيرة الله ومختاره، ألا إن وارث كل علم والمحيط به، إلا إنه المخبر عن ربه عزوجل والمنبه بأمر إيمانه، ألا إنه الرشيد السديد، ألا إنه المفوض إليه، ألا إنه قد بشر به من سلف بين يديه، ألا إنه الباقي حجة ولا حجة بعده، ولا حق إلا معه، ولا نور إلا عنده، ألا إنه لا غالب له ولا منصور عليه، ألا وإنه ولي الله في أرضه وحكمه في خلقه وأمينه في سره وعلانيته

He (P.B.U.H&H.P) said:
Know that the seal of the leaders from among us shall be the Qa’im, the Mahdi.
He shall, most certainly, prevail over (all) religious (methods of life).
He shall definitely be the avenger against the oppressors.
He shall surely be the conqueror of the strongholds and their demolisher.
He shall be, indeed, the destroyer of every polytheistic group.
He shall be the supporter of the religion of Allah.
He shall bring out (people’s share) from (his) deep ocean (of knowledge and divine resources).
He shall mark each man of distinction by his distinction and every man of ignorance by his ignorance.
Behold! He is the elite of Allah, and Allah is his chooser.
He shall be the heir of every knowledge, and the one who shall encompass [every perception]
He shall advise and inform on behalf of his Lord, the mighty and the majestic, and shall remind about the matters pertaining to His faith.
He shall be the right-minded and unerring, to whom authority shall be vested.
Behold! All former (prophets) have given the glad tidings of him.
Know that he shall be the remaining proof (of Allah), after whom there shall be no (new) proof.
There shall be no truth except with him, nor shall there be any light (of guidance) except at his disposal.
None, indeed, shall overcome him, and none shall be supported against him.
He shall be the authorized representative of Allah on His earth, His judge over His creation, and His trustee in His secrets and in what He made evident.
[Source: Ihtijaj – e- Tabarsi, Vol 1/Rawzadatul Waizeen, Part 1]

Ref: Marefate AhleBait (A.S).com