Prophet Muhammed (p) sent one of his Companions named Al-Harith bin Umair Al-Azadi as a messenger to give a letter to the King of Busra. While Al-Harith halted for rest, he was intercepted by Shurabbil Ibn Amr al-Ghassani, and asked where he was going, he (Harith) responded by saying that he had a letter from the Prophet (p) to give to the King of Busra. With the Shurabhil hearing these words from the messenger, he told his men to tie him up and murdered him. The murderer, Shurabbil Ibn Amr al-Ghassani was part of the Byzantine (Roman) empire. Upon the Prophet (p) receiving news, he prepared his companions to engage these people.
“THE RAID OF MU’TA
Al-Waqidi related to us saying: Rabi’a b. Uthman related to me from Umar b. al-Hakam, who said: The Messenger of God sent al-Harith b. Umayr al-Azdi, one of the Banu Lihb, to the King of Busra with a document. When he came down to Mu’ta, Shurahbil b. Amr al-Ghazzani confronted him and said, ‘Where are you going?’ He replied, ‘Al-Sham.’ He said, ‘Perhaps you are one of the messengers from the Messenger of God?’ He said, ‘Yes, I am a messenger from the Messenger of God.’ Then Shurahbil commanded that he be tied with a rope, and executed him. Only he was killed for the Messenger of God.” 
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF MUTAH WHICH IS NEAR AL-BALQA AND IT IS CLOSE TO DAMASCUS.
Then (occurred) the sariyyah of Mutah which is near al-Balqa, and al-Balqa is close to Damascus, in Jumada al-Ula in the eighth year from the hijrah of the Aposte of Allah. They (narrators) said:
The Apostle of Allah sent al-Harith Ibn Umayr al-Azdi, a person from Banu Lihb to the Malik of Busra with an epistle. When he halted at Mutah Shurabbil Ibn Amr al-Ghassani intercepted him, and killed him. No envoy of the Apostle of Allah, besides him, was ever killed. He (prophet) felt deeply grieved and called his people. …” 
Shaykh Muhammad al-Ghazali:
“THE MU’TAH EXPEDITION
The Muslims were grieved at the death of their envoy to the governor of Basra and the ignoble way in which he had been treated. Bubayl ibn Amr had tied his hands over, and pushed him then struck off his head. He was the only envoy to be killed from among the many missions sent by the Prophet. This was felt as a grave insult by the Muslims since envoys are not killed. They therefore determined to take retaliatory measures and teach a lesson to that sinful governor who did what he did on behalf of the Romans.” 
Shaykh Allamah Shibli Nomani:
“Muta is a place in Syria situated on this side of Balqa. The swords, so well-known in Arabia were made here. Kuthair, the famous poet says:
‘The swords that receive a glittering finish at Muta.’
The Prophet (p), had addressed a letter to the king of Busra or the Byzantine Emperor. Among the Arab chieftains who held sway over the border tracts of Syria and Rabi’a, there was one Shurahbil Ibn Amr, ruling over the Balwa region as a vassal of the Hiracleus. This Arab family had long since been converted to Christianity and held territory on the fringes of Syria. Harith Ibn Umair who had gone with the letter was put to death by Shurahbil.
To avenge his murder, the Prophet (p); collected an army of three thousand and despatched it to Syria. Zaid Ibn Haritha, the freed slave of the Prophet (p) was given the command with instructions that Ja’far Tayyar, was to succeed him if he died, and in case Ja’far also fell, Abdullah Ibn Rawaha was to take his place.” 
 The Life of Muhammad: Al-Waqidi’s Kitab Al-Maghazi [Translator Rizwi Faizer], page 372
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, volume 2, page 158
 Fiqh-us-seerah, by Shaykh Muhammad al-Ghazali, page 399
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – volume 2, page 183