Islamic sources state that the Companions were sent out to do missionary work i.e., preaching the message of Islam, inviting people to Islam.
When the Companions reached a place called Dhat Atla (Atlah), they encountered a lot of people, the Muslims started preaching and inviting people to Islam, instead of peacefully responding by declining the offer to accept Islam, and ignore the Companions words, they started attacking the Muslims for preaching.
Hence, a battle ensued with those who started attacking the Muslims. All the Muslims were killed, except one was left alive wounded.
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF KA’B IBN UMAYR AL-GHIFARI TOWARDS DHAT ATLAH BEYOND WADI AL-QURA
Then (occurred) the sariyyah of Ka’b Ibn Umayr al-Ghifari towards Dhat Atlah which lies beyond Wadi al-Qura in the month of Rabi al-Awwal of the eighth year from the hijrah of the Apostle of Allah. … Muhammad Ibn Abd Allah related to me on the authority of al-Zuhri; he said:
The Apostle of Allah sent Ka’b Ibn Umayr al-Ghifari with fifteen men, and they reached Dhat Atlah, in the land of Syria. They encountered there a large concentration of the people. They invited them to embrace Islam, but THEY did not respond and RESORTED TO SHOOTING ARROWS.
When the Companions of the Apostle of Allah observed this, they fought a fierce battle, and were killed. Among the slain was a wounded person, who laboured hard to move in the night, when it was cool, and ultimately reached the Apostle of Allah. He surprised him of what had happened. He was much grieved at it and wanted to send an expedition but he was informed that they had movd to another place; so he abandoned the idea.” 
“In this year a raiding party led by Amr b. Ka’b al-Ghifari went to Dhat Atlah. He set out with fifteen men, and when he reached Dhat Atlah he encountered a large force of men WHOM they SUMMONED TO ISLAM. THEY refused to respond and KILLED ALL OF AMR’S COMPANIONS. He managed to get away and returned to Medina.
According to Al-Waqidi: Dhat Atlah is in the direction of Syria. Its people belonged to [the tribe of] Quda’ah. Their head was a man named Saus.” 
Maulana Muhammad Yusuf Kandhelwi:
“THE DAWAH THAT HADHRAT KA’B BIN UMAYR GHIFAARI GAVE
Zuhri narrates that the Prophet sent Ka’b bin Umayr Ghifaari with a group of fifteen men. They rode to a place in Shaam called Dhaat itlaa where they encountered a very large concentration of people.
When they invited these people to Islaam, they refused and started firing arrows at them. Seeing this, the Companions started fighting them most fiercely but all of them were martyred except one man who was left wounded aming the dead.
When night arrived, he somehow managed to get himself to the Prophet. The Prophet was on the verge of sending a battalion after them when the news reached him that the people had moved on to another place.” 
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, volume 2, page 158
 The History of al-Tabari: The Victory of Islam: Muhammad at Medina A.D. 626-630/A.H. 5-8 (SUNY series in Near Eastern Studies), [Translator:Michael Fishbein], volume 8, page 143
 Hayatus Sahabah (The Lives Of The Sahabah), by Maulana Muhammad Yusuf Kandhelwi, volume 1, page 231