A group of Muslims were sent to Banu Sulaim, led by Ibn Abi Awja Al-Sulami. Their objective was missionary work, preaching. Preaching the message of Islam to this tribe, inviting them to Islam. This tribe didn’t like hearing the preaching of Islam, they started attacking the Muslims. As a result of attacks, being pelted with stones and arrows, the Muslims had to defend themselves. Hence, a battle ensued.
“THE EXPEDITION OF IBN ABI’L-AWJA AL-SULAMI IN DHU-L-HIJJA IN THE YEAR SEVEN AH
Muhammad related to me from al-Zuhri who said: When the Messenger of God returned from Umrat al-Qadiyya in the year seven-he returned in Dhu al-Hijja – he sent Ibn Abi-l-Awja al-Sulami with fifty men to Banu Sulaym. … When Ibn Abi-l-Awja came to them the people were prepared for him. When the companions of Muhammad saw them, and they saw all of them, they invited them to Islam. But they pelted them with arrows and did not hear their words. They said, ‘There is no need for your invitation to Islam,’ and they aimed at them for a while. Helpers came until they surrounded them from every direction. The people fought a terrible battle until most of them were killed. Ibn Abi-l-Awja, their leader was wounded among the dead. He braced himself until he reached the Messenger of God.” 
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF IBN ABI AL-AWJA AL-SULAMI AGAINST BANU SULAYM
Then (occurred) the sariyyah of Ibn Abi al-Awja against Banu Sulaym in Dhu al-Hijjah in the seventh year from the hijrah of the Apostle of Allah. … They (narrators) said: The Apostle of Allah sent Ibn Abi al-Awja al-Sulami with fifty men against Banu Sulaym. He set out towards them. … They assembled, and when Ibn Abi al-Awja arrived there they were ready to meet him. He invited them to embrace Islam. They said: We are not in need of what you invite us to. THEN THEY SHOT ARROWS CONTINUESLY FOR AN HOUR. THEIR SUPPORTERS BEGAN TO POUR IN AND ULTIMATELY THEY SURROUNDED THEM FROM ALL SIDES. The People fought bravely till many of them fell. Ibn Abi al-Awja was wounded while others were slain. Then he strove hard and ultimately reached the Apostle of Allah. This they arrived at al-Madinah on the first day of Safar of the eighth year.” 
“…the expedition of Ibn Abi al-Awja al-Sulami to the land of the Banu Sulaym where he and all of his companions were killed…” 
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 The Life of Muhammad: Al-Waqidi’s Kitab Al-Maghazi, page 365
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, Volume 2, page 153
 The History of al-Tabari: The Last Years of the Prophet – [Translator: Ismail K. Poonawala], volume 9, page 119