” The Prophet (SA) answered...
” The Prophet (SA) answered, “Oh Ali, are you not pleased that I become your brother? ” So Ali said, “Yes, Oh Prophet of God!” The Prophet (SA) replied, “You are my brother in the life of this world and in the hereafter.” Certainly, it is not a coincidence that the Prophet (SA) left Imam Ali (AS) to be the last to assign brotherhood so that his case stands out in front of all the companions and the question is raised as to “Who is the brother of Ali?” One can imagine that no one expected for the Prophet (SA) to declare himself as the ‘brother’ of Ali.
What a great honor for Imam Ali (AS) which speaks volumes of the great status which he enjoys in the eyes of Allah (SWT)! It is not possible for the Prophet (SA) to designate Imam Ali (AS) as his brother without prior divine approval in this regards, so his declaration in the event of brotherhood is certainly a sign and indication of Allah’s pleasure towards Ali and his superiority in His eyes.
Furthermore, it is interesting to note that the purpose of assigning brotherhood in this event was to pair up a companion from the Muhajireen with a companion from the Ansar. However, in the case of Imam Ali (AS) and the Prophet (SA), they do not fulfill the criteria of being from the two groups. This is yet an indication that the brotherhood between them is not random and is rather intentional and without mistake.
What was the need of public declaration of their brotherhood? Certainly, this event signifies the eternal spiritual brotherhood between the two which will translate itself into the position of successorship after the Prophet’s demise. It also highlighted the similarity between the superior character and nature of Imam Ali (AS) as that of the Prophet (AS).
When Imam Ali (AS) was been declared by the Prophet (AS) to be his brother, it proved that he was the only person amongst the Muslims deserving of that position! This choice is not because of the ancestral affinity but it is on the basis of identical qualities of the two individuals.