There is an interesting discussion between Imam Ali Ibn Moosa al-Reza (a.s.) and the tyrant of the time Mamoon al-Abbasi.
Mamoon questioned Imam Reza (a.s.) about the greatest superiority of Ameerul Momineen (a.s.) as proved by the Majestic Quran.
Imam Reza (a.s.) replies – His superiority (over others) in the incident of Mubahelah. Allah said:
‘But whoever disputes with you in this matter after what has come to you of knowledge, then say: Come let us call our sons and your sons and our women and your women and our near people and your near people, then let us be earnest in prayer, and pray for the curse of Allah on the liars.’ (Surah Ale Imran (3): Verse 61)
The Prophet (s.a.w.a.) invited Imams Hasan (a.s.) and Husain (a.s.), the two referred to as his sons. He (s.a.w.a.) invited Fatima (s.a.). She implied ‘his women’. Finally, he (s.a.w.a.) invited Ameerul Momineen (a.s.) referring to his self, according to Allah’s order.”
Then Mamoon said – Is it not that Allah has referred to sons in the plural (الْأَبْنَاءَ) and the Prophet (s.a.w.a.) invited his two sons specifically. And He referred to the women in the plural (النِّسَاءَ) and the Prophet (s.a.w.a.) invited his daughter – only one woman.
Then it is not permitted that invitation to one’s self (نَفْسُهُ) i.e. Prophet (s.a.w.a.) should be construed as an invitation to other than one’s self (Ameerul Momineen (a.s.)). Therefore, there is no excellence for Ameerul Momineen (a.s.) as you claim. (Mamoon meant that self refers to the Prophet (s.a.w.a.) and therefore superiority was for him (s.a.w.a.) and not Ameerul Momineen (a.s.) since he was not the addressee in the verse.)
Imam Reza (a.s.) retorted, “What you have said, O Ameeral Momineen, is not correct –surely the inviter invites other than himself just like the commander commands other than himself. It is not correct that the inviter invites himself, just like it is not so that the commander commands himself. Since the Holy Prophet (s.a.w.a.) did not invite any man for the Mubaahelah except Ameerul Momineen (a.s.), it is established that he is the self of the Prophet (s.a.w.a.) – نَفْسُهُ – as indicated by Allah in His Book and He has commanded as much in the revelation of the verse.”
Mamoon replied – When the reply is given, question is silenced.
• Al-Fusool al-Mukhtaarah, Part 1, p. 38 (Arabic) by Shaikh Mufeed (r.a.)