In the Name of Allah, the most Gracious, the most Merciful
It is generally supposed that Holy Prophet(pbuh) had not expressly designated anyone as his successor in the spiritual and temporal government. Yet his notoin is framed on an incorrect apprehension of fact, for there is abundant testimony that many times the Prophet(pbuh) had openly indicated Ali(as) as his successor.
The first occasion was when he was ordered by Allah to openly and unreservedly invite his kith and kin to Islam. This occasion is called dawat-al-asheera(inviting the relatives to islam). In the words of ravi. Sale, the Holy Prophet(pbuh) said “Allah has commanded me to call you unto HIM; who therefore, among you will be assisting to me herein and become my brother and my vicegerent(“akhí wa wasiyyí wa khalífatí-“)? All of them hesitating and declining the matter, Ali(as) at length rose up and declared that he would be his assistant and threatened those who should oppose him. Prophet Muhammad(pbuh) upon this, embraced Ali(as).
The Prophet put his hand on the back of ‘Ali’s neck and said:”Inna hadha akhhí wa wasiyyí wa khalífatí fíkum, fasma’u lahu wa atí’u – Verily this is my brother, my successor, and my caliph amongst you; therefore, listen to him and obey.”1
The second occasion was at the time of Ali’s conquest of Khayber. The words of the Holy Prophet(pbuh) quite clearly, positively and expressively give his opinion about Imam Ali(as) and his desire to leave him as the guardian and the propagator of his mission.
He said, “You are from me and I am from you, you will inherit me. You are unto me what Aaron was unto Moses. You will be nearest to me on the Day of Judgement and next to me on the fountain of kawthar, enmity against you is enmity against me, as war against you is a war against me. You have as much faith in Allah as I have. You are door to me”.
What more can one say?? Can there be anything more forceful, more eloquent, more pregnant with clear indications and more categorical than the words which the Holy Prophet(pbuh) has used? Do they leave any shadow of doubt?? Has the Holy Prophet(pbuh) ever used such words for anybody else?
The third instance was the occasion of the invasion of Tabuk. To understand the occasion and the cause of remarks of the Holy Prophet(pbuh) it is necessary to know the historical backgound of the incident. It was the summer of 9 A.H. and the Holy Prophet(pbuh) had received information that the Roman king was mobilizing his forces to invade the Islamic State and many Arab tribes were gathering around him. Holy Prophet(pbuh) decided to face them in their own land and not to allow them the run of the Muslim state so that they may not lay waste to the lands through which they would pass.
The situation had become very serious because there was famine in Hijaz, Ta’if and Yemen. The hypocrites were carrying on an intensive propaganda campaign to try to make people believe that the famine was the sign of Allah was angry with Muslims and wants to exterminate them with the worst form of death. And in the case of a defeat against the Roman armies, there was an imminent danger of rebellion.
It was imperative that the state should be left in the hands of a faithful and powerful guardian; otherwise there was every possiblity of being sandwiched between the two enemies(internal and external). The Holy Prophet(pbuh), therefore, decided that Imam Ali(as) should act as a regent in his place so the world might realize that in the opinion of the Holy Prophet(pbuh), none could look after the temporal as well as spiritual welfare of the Muslims during his absence except Ali(as).
He called Ali(as) and told him that he would have to act as the last line of defence for him and Islam saying, “O Ali! Nobody could look after the center of the Muslim state but you or I.” Imam Ali’s stay at Madina disappointed the hypocrites(munafiqun) as they constituted the majority of those left behind by the Holy Prophet(pbuh). They claimed that the Holy Prophet(pbuh) had lost faith in Imam Ali(as) and had, therefore, left him behind and that it was positively certain that the Holy Prophet(pbuh) was going to suffer a defeat. Imam Ali(as) naturally felt anxious for the Holy Prophet(pbuh) and annoyed at aspersions against him.
Imam Ali(as) left Madina immediately, met the Holy Prophet(pbuh) at the place of Jerf and told him all that was said at his back. Thereupon, Holy Prophet(pbuh) said, “Ali! They shall lie against you as they have lied against me. They have called me an epileptic, a magician, a sorcerer and a necromantic and have always portrayed me as a liar. I have appionted you as my vicegerent and my caliph over all those whom I have left behind. Are you not satisfied to realize that you are to me like Aaron was to Moses?”
The fourth and most important time was “Notably the occasion of the return journey from the performance of the Farewell pilgrimage, during halt at a place called Ghadir-e-Khumm, the Holy Prophet(pbuh) had assembled the people accompanying him , using the words which could leave little doubt about his intention regarding his successor, Ali(as). Holy Prophet(pbuh) said, “To me, Ali(as) is what Aaron was to Moses. O Allah! Be the friend of his friends and the foe of his foes; help those who help him and frustrate those who betray him”.
I would be doing a great disservice to the cause of the truth and to the history of Islam if I do not sketch at least an outline of this incident. More than fifty thousand(by some sources One hundred Twenty thousand) people had gathered on that occasion, and many of them narrated all that took place there. Among those narrators we find such luminaries as Abu Bakr, Umar, Othman, Al-Zubair ib Al-Awwam, Abdullah ibn Umar, Abdullah ibn Abbas, Mother of the faithful Aisha, Mother of the faithful Umm Salamah, Abdullah ibn Mas’ud and Hassam ibn Thabit. The book Arjahul-Matalib cites 100 names of the companions of the Holy Prophet(pbuh) [who testify the Ghadir-e-Khumm incident]. Full incident detail of the incident https://hussainicalgary.com/knowledge-base/full-sermon-of-prophet-muhammadpbuh-at-ghadir-e-khumm/.
Original Source: Peak of Eloquence, Nahjul Balagha