A brief look at the Life of Imam Hussain (a.s)
[Zirr b. Hubaysh reported on the authority of Ibn Mas'ud: ]
While the Prophet, may God bless him and his family, was praying, al-Hasan and al-Husayn came and stood behind him. When he raised his head, he took them tenderly (into his arms). When he resumed (his prayers), they resumed (theirs).
Then when he had finished, he sat one on his right knee and the other on his left knee and said: Whoever loves me, should love these two. They, peace be on them, are the two proofs (hujjatay Allah) of God of His Prophet in the contest of prayer (mubahala). After their father, the Commander of the faithful, they were the two proofs of God (hujjatay Allah) to the community concerning religion (din) and belief (milla).
[Muhammad b. Umayr has reported on the authority of his teachers (rijal) that Abu Abd Allah (i.e. Imam Ja'far al-Sadiq), peace be on him, said:]
Al-Hasan, peace be on him, said to his companions: God has two cities - one in the east and the other in the west - in which Gods creatures are never interested in disobeying Him. Yet, by God, God's proof to His creatures, both in those two and between them, is no other than myself and my brother, al-Husayn.
A narration of the same kind is reported from al-Husayn b. Ali peace be on them, when he said to the followers of Ibn Ziyad on the day of deprivation:
What is the matter with you that makes you help each other against me or (don't you see) if you kill me, you kill God's proof to yourselves? No, by God, there is not between Jabilqa and Jabirsa a son of a prophet through whom God provides proof to you other than myself.
He meant by Jabilqa and Jabirsa, the two cities which al-Hasan, peace be on him, had mentioned.
One of the conclusive pieces of evidence (burhan) of the perfection of both of them, peace be on them, and the proof of God's special regard for them both, after what we have mentioned of the contest of prayer of the Prophet, may God bless him and his family, (in which he used) them both, is the pledge (bay'a) which the Apostle of God made to them, when he never made any (other) pledge to a child.
The Qur'an also brings down the positive statement of the reward of Heaven to them both because of their works, despite the outward state of childhood in which they were.