The Prophet of Islam (pbuh) wrote a letter to Abdul Haris bin Alqama, the Grand Bishop of Najran, who was the official representative of the Roman Church in the Hijaz and invited the people of that area to embrace Islam. The text of the said letter runs as under:
In the name the Lord of Ibrahim, Ishaq and Ya'qub
This is a letter from Muhammad, the Prophet and Messenger of Allah to the Bishop of Najran: I Praise and glorify the Lord of Ibrahim, Ishaq and Ya'qub, and invite you all to worship Allah instead of worshipping His creatures, so that you may come out of the guardianship of the creatures of Allah and take place under the guardianship of Allah Himself. And in case you do not accept my invitation you must (at least) pay Jizyah (tribute) to the Islamic Government (in lieu of which it will undertake the protection of your lives and property), failing which you are hereby warned of dangerous consequences.
Muhammad (pbuh), say to the followers of the Bible, "We must come to a common term: 'Let us worship no one except Allah'........" (Surah Aal e Imran, 3:64)

The Prophet (pbuh) goes for Mubahila (Imprecation)

Towards the close of the ninth year of Hijri, embassies from all parts of Arabia came uninterruptedly to the Holy Prophet (pbuh) at Medina, to profess Islam and to declare the adherence of their tribes to Prophet Muhammad (pbuh). (Mention is made in the Holy Qur'an as below about this in Surah An-Nasr (Succor, Divine support)
When there comes the help of Allah and the victory, And you see men entering the religion of Allah in companies, Then celebrate the praise of your Lord, and ask His forgiveness; surely He is oft-returning (to mercy). 110:1-3
Until now, the Christians of Najran (a city in the province of Yemen) had kept themselves aloof. The Holy Prophet (pbuh) sent a letter, inviting them to embrace Islam. In response to that letter the Christians counseled among themselves the course of their action and did ultimately send a representative deputation of fourteen members to Medina to study the facts pertaining to Hazrat Muhammad (pbuh) and his mission. Three Christian scholars, viz. Abdul Maseeh Aaquib, Saiyed and Abdul Haris, headed the deputation.
When these deputies reached Medina, they changed their clothes, which they had worn on the journey, dressed themselves in silken garments, put rings of gold on their fingers, and went to the mosque to greet the Holy Prophet (pbuh). All of them greeted the Prophet traditionally, but the Apostle of God did not respond and turned his face away from them. They left the mosque and approached Usman and Abdul Rahman bin Auf, complaining "your Apostle wrote to us and invited us, but when we went to see him and wished him, he neither reciprocated our wishes nor replied to us. Now what do you advise us to do? Should we go back or wait for another opportunity?" Usman and Abdul Rahman could not comprehend the situation. At last they took the disputes to Ali (as), who advised them to remove the clothes of silk and the rings of gold that they were wearing and to put on their priestly robes. The Holy Prophet (pbuh) would then willingly see them. Thereupon the Christian delegates changed into humble garments and presented themselves to the Apostle who then responded to their salutations and said, "By the Lord who has appointed me His Messenger, when they first came to me they were accompanied by Satan".
Thereafter the Apostle preached to them and requested them to accept Islam. They asked, "What is your opinion about Jesus Christ?" The Apostle said, "You may rest today in this city and after being refreshed you will receive the replies to all your questions from me." The Apostle was awaiting a revelation in this matter, and the next day the verses of the Holy Qur'an Surah Aal-e-Imran verses 59-60 were revealed to him to show the true position of Jesus Christ.
Surely the likeness of Jesus is with Allah as the likeness of Adam; He created him from dust, then said to him 'Be', and he was. (This is) the truth from your Lord, so be not of the disputers. 3:59-60
When they reappeared before the Holy Prophet, The Holy Prophet recited the above verses before the visiting Christians explaining that Christ was a Prophet like Adam and like Adam, created from dust and therefore could not be the son of God. After this, the Holy Prophet invited them to embrace Islam. The Christians remained obstinate and refused to be convinced by anything. Thereupon the following verse No. 61 from Surah Aal e Imran was revealed:
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 ourselves and yourselves, then let us be earnest in prayer, and invoke the curse of Allah on the liars. 3:61
Now the Holy Prophet reproduced the Quranic verse before the deputation of the Christians and declared the challenge of 'Mubahila'. The term 'Mubahila' is derived from its Arabic root 'Bahlah' meaning 'curse'. Thus the word 'Mubahila' literally means cursing each other. The Christians consulted each other and ultimately announced their acceptance of the challenge.
Early next morning the Holy Prophet (pbuh) sent Salman al Farsi ( May Allah be well pleased with him ) to the open place, fixed outside the city for the historic event, to erect a small shelter for himself and those he intended to take along with him for the contest. However, it was on the morning of 24th Zil hijja that the Holy Prophet (pbuh) emerged at the appointed time and place, from his sacred abode with Imam Husain (as) in his arms and holding Imam Hasan's (as) hand and Hazrat Fatima (sa) followed by Hazrat Ali (as). The Holy Prophet then directed them to utter "Ameen" when he prayed to God.
No sooner had the sacred caravan of the Holy Prophet appeared to the sight of the opposing group of the Christians of Najran then they were awestruck and spellbound. Abdul Haris lbne Alqama, the greatest scholar among them, addressed his people:
"Verily I see a divine light on the face of our combatants; I am beholding such faces among them as can make the mountains move from their spots if they pray to God. So beware! Never try to contest with them, otherwise you will perish and the entire nation of Christians will succumb to extinction!"
Thereupon the Holy Prophet reiterated, "By God! Had the Christians of Najran contested with us, they would have been transformed into monkeys and swans. Fire would have rained over them from the sky and they would have been doomed."
When the Christians refrained from 'Mubahila', the Holy Prophet (pbuh) put before them two alternatives: either to embrace Islam or to be prepared to come to terms. But the Christians would not agree till the matter was finally decided by an offer of treaty from their side. Thus a peace treaty was signed on the terms that the Christians of Najran would thereby be committed to pay the Holy Prophet (pbuh) an annual tribute consisting of two thousand costumes worth: forty thousand Dinars, thirty horses, thirty camels, thirty armors and thirty spears.