The Holy Prophet is reported to have said to Imam Ali: "In relation to me you occupy the same position as Harun occupied in relation to Musa with the exception that there will be no prophet after me."
The Holy Prophet said that when he was proceeding for the Tabuk operation, which was only a campaign, not a battle. It took place after the Battle of Mu’tah, which was the last battle fought by the Arabs against the Romans during the lifetime of the Holy Prophet. This battle took place to the east of Medina. Istambul (Constantinople) was the capital of the Eastern Roman Empire. Syria was also under the Romans. Brisk preparations were going on there to launch an attack against Medina. The Holy Prophet deemed it advisable to take troops up to the border of the Romans and he successfully accomplished that mission.
The Holy Prophet, as the politicians put it, wanted to make a show of his power. The Muslims went up to the Roman border and then came back. In this expedition the Holy Prophet did not take Imam Ali with him. He left Ali as his successor in Medina. The Shi’ ah scholars say that this action of the Holy Prophet shows that he knew that fighting was not going to take place. Naturally Imam Ali did not like the idea of being left behind. As he felt dejected, he said to the Holy Prophet:
"Would you not take me with you? Do you leave me here with the women and the children?"
The Holy Prophet said:
"Do you not like to occupy the same position in relation to me as Harun occupied in relation to Musa, except that there would be no prophet after me?"
The Holy Prophet meant to say that whatever relationship Harun bore to Musa, Imam Ali bore to him with the exception of Prophethood. Now let us refer to the Qur’an to find out what relationship existed between Harun and Musa. We find that the Qur’an reports that in the beginning of his mission Musa asked Allah:
"My Lord! Relieve my mind and ease my task for me; and elaborate my tongue, so that they may understand my saying. Appoint for me a Vazir from my folk, Harun, my brother. Strengthen my back with him. And let him share my task, so that we glorify you much, and much remember you." The Holy Qur’an (Surah20, Surah TaHa, verse34)
The word Vazir is derived from the root, Vizr which means a burden or a responsibility. A Vazir is the person who relieves the burden of his master and shares his responsibility. Later this word came into use in the sense of the minister of a king.
Hence, Prophet Musa asked Allah to appoint a person to help him and share his task. For this purpose he suggested the name of Harun.
At another place in the Qur’an we see that Prophet Musa says to Harun:
"Harun, take my place among my people." The Holy Qur’an (Surah7, Surah al-A’ raf (The Wall with Elevations), verse 142)
Thus we find that according to the Qur’an Harun was Musa’s Vazir, his chief supporter, his partner in his task and his successor among his people. That was the relationship between Prophet Musa and Harun and the same should be the relationship between the Holy Prophet and Imam Ali.
Had the Holy Prophet not said, "Except that there would be no prophet after me", We could say that the Holy Prophet had some particular likeness in mind, but when he excluded Prophethood, it became clear that this relationship existed in all other fields (of course social, not physical). It appears as if the Holy Prophet wanted to say to Imam Ali: You occupy the same position in relation to me as Harun occupied in relation to Musa in all divinely appointed fields.
The answer which the Sunnis give to this argument is that they could accept this tradition, if it had been continuous, but it is an isolated one. But as we said earlier, the scholars like Mir Hamid Hussein have proved in their books that this tradition is continuous.