As Harun was to Musa

Sad bin Abu Waqqas narrated from his father that the Prophet (sal Allahu alaihi wa sallam) said to Ali (radi Allahu anhu): "Would it not please you to be to me as Harun was to Musa?" [Sahih Bukhari]

The Messenger of Allah (sal Allahu alaihi wa sallam) spoke these words when he was setting out for the battle of Tabuk, and had charged Ali (radi Allahu anhu) to look after the affairs of Madina in his absence. Ali (radi Allahu anhu) was grieved at being left out of Jihad and said, "Do you want to leave me with children and women?" It was in response to this that the Messenger of Allah (sal Allahu alaihi wa sallam) spoke the words quoted above. [Sahih Bukhari]

Some people have tried to deduce from this hadith proof of Ali's right to be the immediate successor of the Prophet (sal Allahu alaihi wa sallam). Harun (alaihis salam), they argue, was the successor of Prophet Musa (alaihis salam), therefore Ali must also be considered the rightful successor of Prophet Muhammad as khalifa of the community. It is because of this assumption that they question the appointment of the first three khulafa and hold them guilty of usurping the right of Ali (radi Allahu anhu).

This is a clear case of misconception as the commission of Harun to be his brother's deputy was just a temporary arrangement, meant to last only as long as Musa lived. Similarly, Ali's comission to work as the Prophet's deputy during the Prophet's military campaign of Tabuk was for a certain period of time during the lifetime of the Prophet (sal Allahu alaihi wa sallam).

Moreover, Harun (alaihis salam) never succeeded Musa (alaihis salam), as he had already died during the lifetime of his brother. It was in fact Yusha bin Nun who assumed the mantle of Musa (alaihis salam) after his death. Thus, even if the hadith is interpreted as containing a promise of Ali's khilafat, there is no basis to believe that he should be the first khalifa after the Prophet's demise, without anyone intervening in between.