This well proven Hadith in Sunni sources, mentioned in Bukhari , Muslim, Trimdhi, Nissaie and other books. We want to understand this hadith in the light of verses of Holy Quran.
Quran reports about Moses and Aaron. Surah Taaha (20: 25-36)” Moses said, “Lord, grant me courage, Make my task easy, and my tongue fluent, so that they may understand me. Appoint a deputy (for me) from my own family, Aaron, my brother, Strengthen my back by him, And associate him (with me) in my affair, That we may glorify You much, And remember You much, You are Well Aware of our situation.” [Allah] said, “You have been granted your request, O Moses.”
We have mentioned the verses of Quran. So we can understand the position of Aaron to Moses. Now we caneasily understand this hadith ” O Ali; You are to me as Aaron to Moses , the only difference is there is no Prophet after me.”
We can also understand from the verses of Holy Quran that Allah appoints a deputy to prophet. Not even prophet himself or people.
Sayyidina Jabir ibn Abdullah (RA) reported that the Prophet said to Ali, “You are to me like Harun was to Musa, but that there is no Prophet after me.”
Imam Tirmidhi graded the hadith as Hassan(good) Hadith
Here is another narration
Sayyidina Sad ibn Abu Waqqas (RA) reported that the Prophet (SAW) said to Ali “You are to me like Harun was to Musa.” (except that there is no Prophet after me).”
This hadith is graded as Hassan and Sahih