ʿAli Ibn Abū Ṭālib is the cousin of the Messenger (May Allāh exalt his station and grant him peace). Fātimah is the daughter of the Messenger (May Allāh exalt his station and grant him peace), the most prestigious lady in Paradise (May Allāh be pleased with her) as stated by the Prophet (May Allāh exalt his station and grant him peace). Additionally, the Prophet (May Allāh exalt his station and grant him peace) said about ʿAli (May Allāh be pleased with him):
“You are in the same position with relation to me as Aaron (Hārūn) was concerning Moses but with (this explicit difference) that there is no prophet after me.”
He said this when he delegated him the responsibility of [Madīnah] when Muslims were away in the Tabūk conquest. So, he said: O Messenger of Allāh! You leave me behind with women and children? The Prophet (May Allāh exalt his station and grant him peace) said the above narration to warm his heart. It is also because Aaron (Hārūn) took Mūsā’s place among his people, as he said:
“Replace me among my people.” [Sūrah ʾAl-Aʿrāf (7):142]
The previous ʾĀyāt explains the following statement: “You are in the same position with relation to me as Aaron (Hārūn) was concerning Mūsā.”
However, as the Prophet (May Allāh exalt his station and grant him peace) feared that anyone would misuse this word and claim that ʿAli is a Prophet like Hārūn, he said:
“but with (this explicit difference) that there is no prophet after me.” This statement highlights the immense protection the Prophet (May Allāh exalt his station and grant him peace) dedicated to the prestige of the Message [of Islām]. Also, he is concerned about the importance of Tawhid (Islāmic Monotheism). Certainly, according to the Qurʾān and the Sunnah, it is well-known that he is the last of the Prophets, and there is no Prophet after him.