Abu al-Jaarud has narrated that Imam Baqir (a.s.) asked, “O Aba Jaarud! What do the people say to you about Hasan (a.s.) and Husain (a.s.)?”
Abu Jaarud: They do not consider them as the sons of the Messenger (s.a.w.a.).
Imam (a.s.): What did you advance as proof against them?
Abu Jaarud: The verse in which Allah – Mighty and Majestic be He – narrates about Isa Ibn Maryam (a.s.) – and of his descendants, Dawood and Sulaiman and Ayub and Yusuf and Haroun; and thus do We reward those who do good (to others). And Zakariya and Yahya and Isa.. (Surah Anaam (6): Verses 85-86)
Allah has declared Isa (a.s.) from the progeny of Ibrahim (a.s.) even though Isa (a.s.) was born without a father and also there was no verse of malediction (mubaahelah) for him (establishing him as the son of Ibrahim (a.s.) as Imam Hasan (a.s.) and Imam Husain (a.s.) have been established as sons of the Holy Prophet (s.a.w.a.) in the Verse of Mubaahelah in Surah Ale Imran (3): Verse 61).
Imam (a.s.): Then what do they say?
Abu Jaarud: They say that daughter’s son is considered a son but is not from his loins.
Imam (a.s.): O Abu Jaarud, by Allah, I will inform you about one such verse of the Quran with the help of which you can claim that both are the sons of the Messenger of Allah (s.a.w.a.) from the loins and except a disbeliever none can reject it.
Abu Jaarud: May I be held your ransom! Which is this verse?
Imam (a.s.): ‘Forbidden to you are your mothers and your daughters …and the wives of your sons who are of your own loins..’ (Surah Nisa (4): 23)
O Abu Jaarud! You ask them, ‘Were the wives of Imam Hasan (a.s.) and Imam Husain (a.s.) permissible for the Messenger (s.a.w.a.)?’ If they say yes, then by Allah, they are lying and if they say no then by Allah both are the sons of the Messenger of Allah (s.a.w.s.) from his loins. The Messenger of Islam (s.a.w.a.) was not permitted for them (wives of the Imams (a.s.)) only due to the fact that their husbands (a.s.) were from the progeny of the Messenger (s.a.w.a.).
Tafsir-e-Ali b. Ibrahim al-Qummi (r.a.) vol. 1 pg 209; Al-Kaafi, vol. 8, p. 317, H. 501)