Tell those who dispute this with you even after the knowledge that has reached you: “Come, let us gather our sons and your sons, our women and your women, ourselves and yourselves, and pray and solicit God to condemn those who lie.
According to rahyafte (the missionaries and converts website):
The Event of Mubahala was a debate between Islamic prophet Muhammad and the Christians of Najran, where the call was extended to the sons and women. Mubahala is a Qur’anic evocation of the occasion of an encounter between Muslims and Christians during Muhammad’s lifetime. In this occasion Muhammad is instructed by Quran to issue a challenge to the visiting Christians of Najran.
The Christians of Najran came to Medina claiming that Isa (Jesus) is divine and the son of Allah. In fact, they meant to determine whether Muhammad’s claims were in accordance with the prophecies of their holy books. The event is recorded in several Hadith collections and is referred to in the Qur’anic surah Al Imran.
Here, all commentators agree that our women refers to Fatimah al-Zahra (AS), and that this is as the Messenger of Allah (p.b.u.h) wanted it and showed it practically.
In the story of this dialogue which the Prophet (p.b.u.h) commanded with some Christians, the Prophet (p.b.u.h) took a new route to deal with the situation when the dialogue reached a dead end, and it is this method of challenging which the verse tells us about.
The narration of the great Ali bin Ibrahim al-Qommi, from Imam al-Sadiq (AS) says that: ‘The Christians of Najran came (as a delegation) to the Messenger of Allah…. They prayed using the bell; the Prophet’s companion objected: O Messenger of Allah! This in your mosque? He said: Leave them (to pray as they like).
When they finished they came to the Prophet (p.b.u.h) and said: To what do you call? He said: To bear witness that there is no God but Allah and that I am the Messenger of Allah and that ‘Isa (Jesus) (AS) is created a slave (of Allah), and eats, drinks and excretes.
They said: Then who is his father? The revelation was then revealed to the Messenger of Allah (p.b.u.h) saying: Say to them – what do you say about Adam, has he been created a slave (of Allah), eating, drinking, excreting and marrying? So the Prophet (p.b.u.h) asked them and they replied: yes.
He asked them: Then who is his father? They could not answer, so Allah revealed: Verily, the similitude of Jesus with God is as the similitude of Adam; He created him out of dust, then said He unto him BE, and he became’ (Qur’an 3:59) and ‘And unto him who disputeth with thee… let us invoke and lay the curse of God on the liars!’ The Messenger of Allah (p.b.u.h) said: So, challenge me: if I am telling the truth the curse falls unto you, and if I am a liar the curse falls unto me. They said: You are speaking in justice.
They agreed on a date for mubahalah (challenge). When they returned to the places they were staying in, their leaders al-Sayyid, al-‘Aqib and al-Ahtam said: If he challenged us with his people, we accept the challenge for he is not a prophet; but if he challenges us with his family in particular we don’t challenge him, for he is not going to push forward his family unless he is truthful.
In the morning, they came to the Messenger of Allah (p.b.u.h) and with him the Commander of the Faithful, Fatimah, al-Hasan and al-Husain (AS), so the Christians said: Who are those? The people replied: This is his cousin and successor and son-in-law, and this is his daughter Fatimah, and these are his sons al-Hasan and al-Husain. So they became frightened and said to the Messenger of Allah (p.b.u.h): We give you the satisfaction, so give us leave from the challenge.’
source : shafaqna
