The question of Wilayat or leadership of the Ummah after the Holy Prophet (peace be upon him and his progeny), both in spiritual and in temporal affairs, is the fundamental point of division between Shias and Sunnis. Both agree on the indispensable need for a Wali, but differ in the ways of his selection or appointment. Did the Prophet leave the Muslims without appointing anyone as his successor? Do Muslims have the right to elect their own leader as the Prophet's successor? Does the right of appointment of a Wali for the Muslims rest with Allah? Such questions form the basic difference between the two sects of Islam.
Martyr Ayatollah Sayyid Baqir al-Sadr has discussed three possibilities in his essay entitled "A Study on the Question of Al-Wilaya". The first possibility being that the Prophet left the Ummah without any successor after him, the second one being that he wanted the Muslims to choose their leader for themselves through Saqifa or consultation, and the last one being that the Prophet appointed Imam Ali (peace be upon him) as his successor – the Imam of Ummah.
The Prophet was assuming the leadership of a revolutionary faith, inducing radical transformations of the customs, structures, and concepts of society. The path for such transformation was long and the faith which the Prophet practiced was immaculate. He had to deal with people belonging to the Age of Ignorance and raise them to new institutions, thus converting them into believing men and women who could carry the new light and uproot the twisted tree of ignorance from hearts and minds. The Prophet made astonishing headway in the task of transformation in a very short time, but it was necessary for this task of transformation to continue on its way even after the death of the Prophet .
So to think that Prophet Muhammad adopted a passive approach for leadership after him is to attribute a lack of insight to the Prophet. To think such a thing is a great sin – besides, all logic and reason with many evidences refute such an attribution. The Prophet was well aware of the maturity of the faith of the Muslims who had just emerged from the dark alleys of pre-Islam ignorance, and they were certainly not in a condition to cope with the vacuum of leadership. There were also many hypocrites present in the garb of Muslims, and the Prophet also knew it well.
The Prophet was aware of the dangers to Islam after him, and hence claims of any kind of passivity on his part for the future of Islam is not acceptable. In fact, both Shi'a and Sunni narrators have this tradition recorded in their authentic works that Umar was with a group of men in the house when the Prophet, before his death, said: "Bring me parchment and pen so that I may write something for you after which you shall never go astray."
In fact, this act of the Prophet illustrates clearly that he was deeply concerned about the dangers which had to be faced in the future. It also shows that he recognized the need to plan ahead so as to protect the Ummah from deviation and save it from inattentiveness and disintegration. It is thus totally impossible to substantiate any claim of passivity leveled against the Prophet.
The second approach says that the Prophet adopted a positive policy concerning the future of the Ummah and Islam, and he advocated the appointment of a Shura or counsel, which would be entitled to select a leader of theMuslims.
Had the Holy Prophet adopted a positive attitude towards Shura after him, he would have found it absolutely necessary to educate the Ummah and the faithful concerning the principles of Shura with its rules and details, while also mentally and spiritually preparing the Islamic society to accept this system. But we can easily discover that the Holy Prophet never sought to educate the Ummah for any such plans for future. We do not find any narrations attributed to the Prophet giving details of principles, legal aspects or theoretical concepts of shura. Therefore, we can conclude that holy Prophet did not seek Shura for the Ummah after his death.
The Shura, which was formed after the Prophet's death, could not survive for even a short time. The first Caliph neglected it and appointed the second Caliph himself, while the second one formed a six-member group enshrining the right to elect a leader amongst them. In this way, Saqifa collapsed immediately after it was formed. The first Caliph regretted the fact that he was elected too speedily, while the second one regretted the first one's election saying that it was a slip, and on many occasions he himself confessed his inability in handling the affairs of Ummah.
The third approach is the only possibility, which is consistent with the nature of facts and is logical in light of the circumstances surrounding Islam, Muslims and the Prophet . The Holy Prophet adopted a positive stance towards the future of Islam after his death, and as per the orders of Allah he chose Imam Ali who had a deep involvement in the formation of Islam. Prophet Muhammad specifically prepared the Imam so that he could exemplify the intellectual authority and political leadership of the experience, and maintain the leadership of the Ummah and its ideological structure after the holy Prophet's death.
Imam Ali himself described his unique relationship with the Holy Prophet: "You know of my connection with the Messenger of Allah , my close kinship to him and my intimate position. He put me on his lap when I was a child, hugged me to his breast, embraced me in his bed, so that his body touched mine and so I smelled his scent, and would also chew things and give them to me to eat. But he did not find me lying in my speech or pompous in my act. I used to follow him as the small camel follows its mother and every day he showed me part of his moral acts and ordered me to do likewise. Every year he used to take me to Hira and only I could see him, for at that period of time of Islam there was only the Messenger of Allah, Khadija and myself as the third, as nobody else lived in the house. So I saw the light of revelation and the message and smelled the fragrance of prophecy."
There are a great many indications from the lifetime of Imam Ali after the death of the Prophet which reveal the Prophet's private ideological training of Imam Ali and reflect the effects and results of this private instruction. The Imam was the man to whom the ruling leadership resorted for consultation and authority when they wished to solve some difficult problems, which they could not solve themselves. But we cannot find a single instance in the history of the Islamic experience during the time of the four caliphs in which the Imam turned to someone else for an opinion as to the way in which a problem should be dealt with according to Islam. On the other hand, there were tens of instances in which the ruling Islamic leadership felt it necessary to consult the Imam in spite of their reservations towards him.
The above arguments along with the event of Ghadeer establish the fact that the successor of Prophet Muhammad was none other than Imam Ali. The Holy Prophet himself, on the instructions of Almighty Allah, appointed him as his successor.
The above quotes and points are from A Study on the Question of Al-Wilaya by Sayyid Muhammad Baqir as-Sadr, translated by Dr. P. Haseltine.