After the death of Mu'awiyah in Rajab 60 A.H., Yazid attempted to exact allegiance from Imam Husayn (A) in order to secure undisputed possession of the Caliphate.
The Imam was the rightful successor to the Holy Prophet (S) and could never have given allegiance to a man as corrupt as Yazid. The Imam left Madinah with his family, and headed towards Makkah. While in Makkah, the Imam was informed that Yazid had sent soldiers to assassinate him. In order to avoid bloodshed and preserve the sanctity of the Haram of Makkah, the Imam left Makkah for Kufah, on the 8th of Dhu al-Hijjah. Dhu al-Hijjah being the month of Hajj, Makkah was filled with pilgrims, and the Imam delivered the following sermon at Mina, before his departure:
O people, take lesson from the counsel God gave to His friends when He rebuked the rabbis by saying: "Why do the scholars and rabbis not forbid their sinful talk and consumption of what is unlawful? Truly what they have done is evil." (Quran, 5:63)
And God says: "Cursed by the tongue of David and Jesus, son of Mary, are those among the Children of Israel who disbelieved on account of their rebellion and transgression. They did not prevent each other from committing vile and corrupt acts; surely what they did was abominable!" (Quran, 5:78-79)
God reproached them because they saw with their own eyes the oppressors committing vile and corrupt acts, but did not stop them, out of love for the favors they received from them as well as fear of persecution and injury. However, God says: "Fear not men, but fear Me." (Quran, 5:44) And He says: "The believing men and women are friends and protectors to each other; they enjoin the good and forbid the evil (they perform the prayer, and pay the alms, and obey God and His messenger. They - upon them God shall have mercy; God is Almighty, All wise)." (Quran, 9:71)
God mentions the duty of enjoining the good and forbidding the evil (al amr bi al-ma'ruf wa al-nahy 'an al-munkar) over all other duties, because He knows that if it is performed and is established in the society, all other duties, the easy and the difficult, will also become established. The reason for this is that al-amr bi al-ma'ruf wa al-nahy 'an al-munkar means summoning people to Islam, as well as resistance against injustice, opposing and struggling against oppressors, and endeavoring to ensure that public wealth and income derived from war are distributed in accordance with the just laws of Islam, and that taxes are collected, levied, and expended in due and proper form.
O scholars, who are celebrated and enjoy good repute on account of your knowledge! You have achieved a good name in society because of your good will. It is on account of God that men venerate you and stand in awe of you, so that even the powerful fear you and the weak honor you, and those who are not subject to you and over whom you hold no authority grant you favors they deny themselves. When the people do not receive their due, they seek your intercession, and you walk in the street with the majesty of kings and princes. Have you not earned all this respect and prestige because of the people's hopes that you will implement God's laws, even though in most instances you have failed to do so?
You have taken lightly your duties as leaders. You have neglected the rights of the oppressed and the lowly, but have assiduously pursued what you regard as your personal rights. You have not spent your money or risked your life for the sake of the One Who has given you life, nor have you fought against any group or tribe for the sake of God.
Nevertheless, you desire - and regard it as your due - that He should grant you paradise, the company of the Prophet, and security from His chastisement in the Hereafter. You have such expectations of God; I fear that the full weight of His wrath will descend upon you, for although it is by His might and glory that you have achieved high rank, you show no respect to those who truly know God, while you yourself enjoy respect among God's creatures on His account.
Moreover, as you see the covenant enacted with God  being violated and trampled underfoot, yet you show no anxiety. When it comes to the covenant enacted with your fathers, you become greatly disturbed and anxious if they are violated in part, but the pledges you have given to the Most Noble Messenger  are a matter of complete indifference to you.
The blind, the dumb, and the chronically ill everywhere lack protection in towns and no mercy is shown them. But you neither behave in accordance with your function and rank, nor you support or pay any regard to those who do. You purchase your safety from the oppressive ruling powers with flattery, cajolery, and compromise.
All these activities have been forbidden to you by God, and He has, moreover, commanded you to forbid each other to engage in them, but you pay no attention. The calamity that has befallen you is greater than what has befallen others, for the true rank and degree of religious scholars has been taken away from you. The administration of the country and the issuing of decrees and ordinances should actually be trusted to religious scholars who are guardians of God's ordinances concerning what is permitted and what is forbidden. But your position has been usurped from you, for no other reason than that you have abandoned the truth and have disagreed about the nature of the tradition of our holy prophet (S), despite the existence of clear proofs.
Had you the forbearance to endure adversities and hardships for the sake of God, then all proposed regulations (God's affairs) would be brought to you for your approval and for you to issue; authority would lie in your hands. But you allowed the oppressors to take away your functions and God's affairs to fall into their hands so that they administer them by resorting to ambiguities. They make arbitrariness and the satisfaction of lust their consistent practice.
What enabled them to gain control of government was your fleeing in panic from (inevitable) death and your love of life, which shall in all certainty depart from you. As a consequence of that mentality, you have delivered the powerless masses into the clutches of the oppressors. While some cringe like slaves under the yoke of the oppressors, and others have been reduced to destitution in regard to their livelihood, the rulers run the affairs of the government in accordance with their whims, earning ignominy and disgrace for themselves with their licentiousness, following evil counselors and showing impudence toward God.
One of their appointed spokesmen mounts the pulpit in each city. The country is defenseless before them, and their hands grab freely whatever they want of it. The people are their slaves and are powerless to defend themselves. One of the governors is a dictator by nature, malevolent and rancorous; another represses his wretched subjects ruthlessly; and still another refuses in his absolutism to recognize either God or the Day of Resurrection! It is not strange - how can one think it strange - that society is in the clutches of a cunning oppressor whose tax collectors are oppressors and whose governors feel no compassion or mercy toward the believers under their rule.
It is God who will judge concerning what is at dispute among us and deliver a decisive verdict concerning all that occurs among us.
O God! You know that everything we did was not prompted by rivalry for political power, nor for such a search for wealth and abundance; rather it was done to demonstrate to men the shining principles and values of Your religion, to reform the affairs of Your land, to protect and secure the indisputable rights of Your oppressed servants, and to act in accordance with the duties You have established and the norms, laws, and ordinances You have decreed.
So (O scholars of religion!) You are to help us reach this goal, win back our rights from those powers who have considered it acceptable to wrong you and who have attempted to put out the light kindled by your Prophet.
God suffices us - upon Him do we rely, to Him do we turn, and to Him shall we return.
 i.e. the social contracts that establish the institutions of society
and determine social relations in Islam.
 i.e. Islamic relationships based upon the oath of loyalty sworn to the
Prophet and the similar pledge to obey and follow his successors, 'Ali
(A) and his descendants, given to the Prophet at the pool of Khum.
1. Al-Tawhid, Vol VII, No. 4, Shawwal - Dhu al-Hijjah: