The expedition of Bir Maona took place because one of the chief’s of the tribe of Bani Amir came to Madinah and requested the Prophet (p) to send some Muslims to his tribe Najd, so they may learn more about Islam. Although he was not Muslim, he was interested in knowing more about Islam before he decided to convert.
The Prophet (p) was hesitant in sending his people because before this, this same tribe had acted treachersly towards the Muslims. However, Abu Bara assured Prophet (p) that nothing would happen to them and guaranteed their safety. The Prophet (p) agreed to send some Muslims to the tribe so they may learn more about Islam.
In order for their safety to be guaranteed a letter was written by the chief leader of Najd for them to have safe passage through. However, one thing that was not mentioned or known among the Muslims in Madinah there was dispute in Najd. Amir bin Tufayl the nephew of Abu Bara wanted to be leader over Abu Bara.
Anyhow, those Muslims were sent out on this mission to teach people the Message of Islam. On the way it is reported that they halted at a place called Bir Ma’una. One of the Muslims was sent to deliver the letter of safe passage to Tufayl bin Amir, arrogant Amir didn’t even read the letter. He rushed and killed the Muslim man who delivered the letter.
Straight after this, he rushed out and incited other tribes to go and kill rest of the missionaries. Everyone got killed except for one Muslim who was left alive and went back to Madinah to tell what had happened.
Musnad Ibn Hanbal:
2746. It was narrated that Ibn Abbas said: The Messenger of Allah prayed qunoot every day for an entire month, at Zuhr, Asr, Maghrib, Isha, and Fajr, at the end of every prayer. When he said, ‘Sami Allahu liman hamidah’ in the final Rak’ah, he would pray against them, against a clan of Banu Sulaim, against Ri’l, Dhakwan and Usayyah, and those who were behind him would say Ameen. He sent (envoys) to them, to call them to Islam, and they killed them. …” (Musnad Ibn Hanbal, vol. 2. pg. 584) 
Kitab al-Maghazi – Ma’mar Ibn Rashid:
The man called Mula’ib al-Asinnah, ‘Lover Of Spears’, came to the Prophet bearing a gift. The Prophet explained Islam to him, but he refused to become a Muslim. … The man replied, ‘Then send whomever you wish to the inhabitants of Najd, and I shall guarantee their safety.’ So the Prophet sent a group. Al-Mundhir Ibn Amr, who was called A’naqa Liyamut, ‘He who hastens towards death,’ was among them and so was Amir Ibn Fuhayrah. Amir Ibn al-Tufayl attempted to muster an army from the Amir clan against the Muslims, but thy refused to heed him and refused to violate the pact of Mula’ib al-Assinah. So Amir Ibn al-Tufayl sought to muster an army from the Sulaym clan, and they heeded his call and pursued the Muslims with nearly a hundred archers. They caught up with the Muslims at Bi’r Ma’unah, where they slew them all save Amr Ibn Umayyah al-Damri, whom they allowed to flee. (Ma’Mar Ibn Rashid’s Kitab al-Maghazi Pg. 100 – 111) 
The Prophet invoked evil upon those (people) who killed his companions at Bir Mauna for 30 days (in the morning prayer). He invoked evil upon (tribes of) Ril, Lihyan and Usaiya who disobeyed Allah and His Apostle. Allah revealed a Qur’anic Verse to His Prophet regarding those who had been killed, i.e. the Muslims killed at Bir Ma’una, and we recited the Verse till later it was cancelled. (The Verse was:) ‘Inform our people that we have met our Lord, and He is pleased with us, and we are pleased with Him.” (Sahih al-Bukhari volume 5, Book 59, Hadith 421)
The Prophet sent seventy men from the tribe of Bani Salim to the tribe of Bani Amir. When they reached there, my maternal uncle said to them, “I will go ahead of you, and if they allow me to convey the message of Allah’s Messenger (it will be all right); otherwise you will remain close to me.” So he went ahead of them and the pagans granted him security But while he was reporting the message of the Prophet, they beckoned to one of their men who stabbed him to death. My maternal uncle said, “Allah is Greater! By the Lord of the Ka`ba, I am successful.” After that they attached the rest of the party and killed them all except a lame man who went up to the top of the mountain. (Hammam, a sub-narrator said, “I think another man was saved along with him).” Gabriel informed the Prophet that they (i.e the martyrs) met their Lord, and He was pleased with them and made them pleased. We used to recite, “Inform our people that we have met our Lord, He is pleased with us and He has made us pleased ” Later on this Qur’anic Verse was cancelled. The Prophet invoked Allah for forty days to curse the murderers from the tribe of Ral, Dhakwan, Bani Lihyan and Bam Usaiya who disobeyed Allah and his Apostle. (Sahih al-Bukhari volume 4, Book 52, Hadith 57)
Hajjah Amina Adil
EVENTS OF BIR MA’UNA
In the fourth year of the Hijra in the month of Safar, Abu Bara Amir bin Malik bin Ja’far, the chief of the tribe of Bani Amir of the Najd came to Madinah to visit the Holy Prophet. The Prophet invited him to enter Islam, and he was not averse to it, but he said, ‘Send some Muslims to my tribe to instruct them in this new teaching.’ The Holy Prophet said, ‘I do not trust the people of the Najd. They may do harm to my friends, and my companions are a trust given to me by Allah. I am responsible for their lives. Your nephew Amir bin Tufayl has spoken such a word against me:
‘Let the deserts be yours, and the cities be ours’. In these words is hidden treachery.’
Abu Bara assured the Holy Prophet Repeadetly that he would guarantee the Muslims safety and that he woud give them a letter of safe passage. Finally the Prophet ordered that a letter be written to Amir bin Tufayl bin Malik, the nephew of the chief of the Bani Amir Abu Bara. It was not known in Madinah that within the tribe there was a dispute about the leadership, and that Abu Bara’s nephew aspired to become the chief in his place. So, a deputation of Muslims was sent forth under Mundhir bin Amr, comprising seventy God-fearing, abstemious men, of the most exemplary among the Muslims in piety and knowledge, who made a living by gathering wood by day and selling it in the evening. Seventy men whose lives were firmly established in the faith were sent out to the Najd.
These noble personages went as far as the watering place called Bir Ma’una where they halted. The Prophet’s letter was given to Haram bin Milhan to deliver to Amir bin Tufayl. Before even looking at the letter he had been handed, this unbeliever rushed to kill the Prophet’s precious friend.
Then he tried to incite the rest of the tribe to go out and kill the whole delegation of Muslims, but mostly they refused to violate the promise of security, which Abu Bara had given these men. Therefore, this accused unbeliever appealed to the tribes of the Banu Sulaym of Usayya and Dhakwan who had recently had hostilities with Madinah, gathering a force of men, they rushed on that group of pious men at Bir Ma’una who had come to instruct them in the Unity of Allah and to teach them about resurrection of death. All of them were massacred, but for one man who was left for dead, Ka’b bin Zayd of the Bani Najjar. This man was picked up from among the slain and eventually made his way back to Madinah.
Two men were out pasturing the camels at the time of this massacre, Amr bin Umayya al-Damri and Mundhir bin Muhammad bin Uqba, and thus they survived the slaughter. When they came to the scene of the disaster and saw what had happened, Mundhir Ibn Muhammad asked his companion Amr for his opinion on what they should do. Amr said,
‘We ought to return to Madinah and inform the Holy Prophet of this state of affairs.’ But Mundhir bin Muhammad said, ‘I wish to remain here with my martyred brothers, I too wish to die a martyr and return to Allah forth-with!’
With these words he threw himself upon the enemies of God and His Holy Prophet and fought until he fell a martyr. Amr was taken prisoner, but Amir bin Tufayl set him free after learning his name and lineage, for he said, ‘My mother has vowed to free a slave. On this account I set you free.’ This noble person returned safely to Madinag. When the Beloved of Allah learned of what had befallen at Bir Ma’una, he was greatly aggrieved and distressed, in fact it may be said that no other event affected him as strongly as this. For one whole month, he invoked against the treachery of these unbelievrs at the end of each prayer. It was not long before each and every one of them came to gried and was plunged into the pit of eternal damnation. 
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 English Translation of Musnad Imam Ahmed Bin Hanbal, volume 2, Page 584
 The Expeditions (‘Kitab al-Maghazi’) – An Early Biography Of Muhammad By Ma’mar Ibn Rashid [Translated: Sean W. Anthony – Copyright 2014 by New York University], page 110, 111
 Muhammad, the Messenger of Islam: His Life & Prophecy By Hajjah Amina Adil, page 356 -358