Let them pray.
Overruling the Companions' RA impulse to stop the Christian delegation of Najran from praying, facing east, inside the Masjid itself.
In brief
Sixty Christians of Najran, bishops and nobles among them, came to Madinah in 9 AH in answer to the Prophet ﷺ's letter, and he let them pray facing east in his mosque. They asked who Isa's father was if he had none; the opening of Surah Ali Imran came down the next day, likening Isa to Adam, and denying that Ibrahim was a Jew or a Christian. Then the verse of mubahala was revealed, and the Prophet ﷺ came out with Fatima, Ali, Hasan and Husayn to invoke Allah's curse on the liars. The Christians, afraid he was telling the truth, declined and accepted the jizya, and asked for Abu Ubaydah as a judge. On the road home their patriarch admitted to his brother that this was the promised prophet, and that he would not give up his honour from Rome to follow him; the brother went back and became a Muslim.
This is the occasion of revelation for the opening of Surat Ali-Imran and for the Ayat al-Mubahala, and it demonstrates the Prophet ﷺ's tolerance toward Ahl al-Kitab, allowing them to worship inside the Masjid itself and, after the Najran delegation ultimately refused Islam, sending them home in peace with their treaty and nothing more done to them. (Umar RA later ended their jizya arrangement and had them relocate out of the Arabian Peninsula during his own reign.)
Other events recorded in 9 AH. Their order within the year is not established; use the storyline above for the sequence.
Sixty Christians pray facing east in the mosque.
Let them pray.
Overruling the Companions' RA impulse to stop the Christian delegation of Najran from praying, facing east, inside the Masjid itself.
Who was Isa's father? Was Ibrahim a Christian?
If you agree with us that Isa was born of a virgin, then who is his father?
I don't have an answer for you now. Let Allah answer you.
إِنَّ مَثَلَ عِيسَىٰ عِندَ اللَّهِ كَمَثَلِ آدَمَ ۖ خَلَقَهُ مِن تُرَابٍ ثُمَّ قَالَ لَهُ كُن فَيَكُونُ
Indeed, the example of Jesus to Allah is like that of Adam. He created him from dust; then He said to him, "Be," and he was.
Was Ibrahim AS a Christian or a Jew?
مَا كَانَ إِبْرَاهِيمُ يَهُودِيًّا وَلَا نَصْرَانِيًّا وَلَٰكِن كَانَ حَنِيفًا مُّسْلِمًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ
Ibrahim was neither a Jew nor a Christian, but he was one inclining toward truth, a Muslim submitting to Allah. And he was not of the polytheists.
فَمَنْ حَاجَّكَ فِيهِ مِن بَعْدِ مَا جَاءَكَ مِنَ الْعِلْمِ فَقُلْ تَعَالَوْا نَدْعُ أَبْنَاءَنَا وَأَبْنَاءَكُمْ وَنِسَاءَنَا وَنِسَاءَكُمْ وَأَنفُسَنَا وَأَنفُسَكُمْ ثُمَّ نَبْتَهِلْ فَنَجْعَل لَّعْنَتَ اللَّهِ عَلَى الْكَاذِبِينَ
Then whoever [still] argues with you about it after [this] knowledge has come to you, say, "Come, let us call our sons and your sons, our women and your women, ourselves and yourselves, then supplicate earnestly [together] and invoke the curse of Allah upon the liars [among us]."
Revealed at the end of the second day of debate: the Ayat al-Mubahala.
We decline the mubahala and accept the condition of jizya.
Deliberating beforehand, they had said among themselves: "If he is a prophet and we do this curse, we shall be obliterated from the face of this earth."
Can you give us a judge to resolve our internal affairs? We see you to be honest people.
Do not curse him: he is the prophet we were promised.
Do not curse him.
Rebuking his younger brother, who had just spoken disparagingly of the Prophet ﷺ on their journey back to Najran.
Why?
Because he is the prophet foretold in our scripture.
Why didn't you accept him then?!
Do you wish to give up all of the honor, wealth, and ties that the emperor has given us?
Shocked by his brother's admission, embraced Islam and returned to Madinah.
إِنَّ مَثَلَ عِيسَىٰ عِندَ اللَّهِ كَمَثَلِ آدَمَ ۖ خَلَقَهُ مِن تُرَابٍ ثُمَّ قَالَ لَهُ كُن فَيَكُونُ
“Indeed, the example of Jesus to Allah is like that of Adam. He created him from dust; then He said to him, 'Be,' and he was.”
Revealed in answer to the Najran delegation's question about the fatherhood of Isa ibn Maryam.
مَا كَانَ إِبْرَاهِيمُ يَهُودِيًّا وَلَا نَصْرَانِيًّا وَلَٰكِن كَانَ حَنِيفًا مُّسْلِمًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ
“Ibrahim was neither Yahudi nor Nasrani. Rather, he was a pure hanif that was a Muslim; and he was not of the pagans.”
Revealed correcting the Najran delegation's claim that Ibrahim AS was a Christian.
فَمَنْ حَاجَّكَ فِيهِ مِن بَعْدِ مَا جَاءَكَ مِنَ الْعِلْمِ فَقُلْ تَعَالَوْا نَدْعُ أَبْنَاءَنَا وَأَبْنَاءَكُمْ وَنِسَاءَنَا وَنِسَاءَكُمْ وَأَنفُسَنَا وَأَنفُسَكُمْ ثُمَّ نَبْتَهِلْ فَنَجْعَل لَّعْنَتَ اللَّهِ عَلَى الْكَاذِبِينَ
“Then whoever [still] argues with you about it after [this] knowledge has come to you, say, 'Come, let us call our sons and your sons, our women and your women, ourselves and yourselves, then supplicate earnestly [together] and invoke the curse of Allah upon the liars [among us].'”
The Ayat al-Mubahala, revealed at the end of the second day of debate with the Najran delegation.
“Do not argue with the People of the Book unless gracefully, except with those of them who act wrongfully. And say, "We believe in what has been revealed to us and what was revealed to you. Our God and your God is One. And to Him we submit."”
Classical tafsir (Ibn Abi Hatim from Ibn Abbas) reports this verse on gracious debate with the People of the Book was revealed regarding the Christian delegation of Najran.
قُلْ إِن كُنتُمْ تُحِبُّونَ اللَّهَ فَاتَّبِعُونِي يُحْبِبْكُمُ اللَّهُ وَيَغْفِرْ لَكُمْ ذُنُوبَكُمْ
“Say, 'If you truly love Allah, then follow me. Allah will love you and forgive your sins.'”
Aali Imran 3:31 falls within the opening block of Surah Aali Imran (v.1-~80) that Ibn Ishaq's Sira explicitly ties to the Najran Christian delegation's debate over Isa/Christian doctrine.