Prophet Muhammed (p) learned that Khalid b. Sufyan al-Hudhali (or Sufyan Ibn Khalid) was inciting and collecting people at Nakhlah (or Urana) to attack the Muslims in Madinah. So, the Prophet (p) called Abdullah Ibn Unays to go to Nakhlah (Or Urana) on a mission to deal with Khalid, if he intended to attack Madinah.
Upon this, Abdullah b. Unays asked the Prophet (p) for Sufyan’s description, Muhammed (p) described Sufyan to him. Abdullah B. Unays reached the territory where Sufyan was.
When Abdullah b. Unays eventually met Sufyan face to face, he asked him if he intended attacking the Muslims. Sufyan replied affirmatively that he was on a mission to kill Muslims. Upon Abdullah b. Unays hearing what Sufyan was going to do, he killed him.
“The Apostle of God send me to Khalid b. Sufyan al-Hudhali. This was towards ‘Uranah and ‘Arafat. He (the Prophet) said: Go and kill him. I saw him when the time of the afternoon prayer had come. I said: I am afraid if a fight takes place between me and him (Khalid b. Sufyan), that might delay the prayer. I proceeded walking towards him while I was praying making a sign. When I reached near him, he said to me: Who are you?
I replied: A man from the Arabs; it came to me that you were gathering (an army) for this man (i.e. the Prophet). Hence I came to you in connection with this matter. He said: I am (engaged) in this (work).
I then walked along with him for a while.; when it became convenient for me, I dominated him with my sword until he became cold (dead).” (Abu Dawud Book 002, Hadith Number 1244)
Kitab al-tabaqat al-Kabir – Ibn Sa’d:
“SARRIYAH OF ABD ALLAH IBN UNAYS
Then (occurred) the sariyyah of Abd Allah Ibn Unays against Sufyan Ibn Khalid Ibn Nubayh al-Hudhali at Uranah. He set out from al-Madinah on 5 Muharram after the commencement of the thirty-fifth month from Hijrah of the Apostle of Allah.
It was because (a report) had reached the Apostle of Allah, that Sufyan Ibn Khali al-Hudhali, later of Al-Lihyan, used to halt at Uranah and adjoining places and was mobilizing the men of his tribe, and others (to fight against the Apostle of Allah). Thereupon the Apostle of Allah, sent Abd Allah Ibn Unays to kill him.
He (Ibn Unays) said: O Apostle of Allah! Describe him before me. He said: When you will see him, you will be frightened and bewildered and you will recall Satan. He said: I am not afraid of men …
Then I took my sword, and went out, pretending to be one of the Khuza’ah, till I entered Unranah. There I met him with his followers from different tribes, and those who had joined him. I recognized him by the description (given by) the Apostle of Allah.
I feared him and I found myself perspiring. … He asked: Who is this man? I said: A man from the Khuza’ah, I heard about your mobilizing the people against Muhammad, so I came to you to join. He sai: Yes, I have been mobilizing (forces) against him. Then I walked with him and talked to him, and he enjoyed talk till I reached his tent. His companions dispersed, were separated from him and went to sleep; then I killed him when he was unprepared.” 
“ABDULLAH B. UNAYS’S RAID TO KILL KHALID B. SUFYAN B. NUBAYH
The Apostle sent him against Khali, who was in Nakhla or Urana collecting men to attack the Apostle, an he killed him. … The Apostle called me and said that he had heard that Ibn Sufyan b. Nubayh al-Hudhali was collecting a force to attck him, and that he was in Nakhla or Urana and that I was to go and kill him. I asked him to describe so that I might know him, and he said, ‘If you se him he will remind you of Satan.
A sure sign is that when you see him you will feel a shudder,’ I went out girding on my sword until I came on him with a number of women in a howdah seeking halting-place for them. It was the time for afternoon prayer, and when I saw him I felt a shuddering as the Apostle had said. I advanced towards him fearing that something would prevent my praying, so I prayed as I walked towards him bowing my head.
When I came to him he asked who I was and I answered, ‘An Arab who has heard of you and your gathering a force against this fellow [Muhammed] and has come to you.’ He said, ‘Yes I am doing so.’ I walked a short distance with him and when my chance came I struck him with my sword an killed him…” 
“The Messenger of God sent Abdullah b. Unays to Khalid b. Sufyan b. Nubayh al-Hudhali while he was in Nakhlah or Uranah preparing to attack the Messenger of God and killed him. …
The Messenger of God called me and said, ‘It has reached me that Khalid B. Sufyan b. Nubayh al-Hudhali is gathering a force to attack me.
He is either in Nakhlah or Uranah, so go to him and kill him.’ I replied, ‘O Messenger of God, describe him to me so that I might know him.’ He said, ‘When you see him he will remind you of Satan. [A sure] sign between you and him is that when you see him you will feel a shudder.’ I went out, girding on my sword, until I came to him while he was in howdah with the women, seeking a halting place for them at the time for afternoon prayer.
When I saw him I found him to be as the Messenger of God had described. I advanced toward him, but fearing that there might be acrimony between me an him which would distract me from the prayer, I prayed, making gestures with my head a I walked toward him.
When I got to him he asked who I was, and I replied, ‘An Arab who has come to you because he has heard about you and your gathering [a force] against this fellow [i.e., Muhammed]. He said, ‘Yes I am doing that.’ I walke a short distance with my sword and killed him. …” 
Kitab Al-Maghazi – Waqidi:
“THE EXPEDITION OF ABDULLAH B. UNAYS TO SUFYAN B. KHALID B. NUBAYH
Abdullah b. Unays said: I set out from Medina on Monday the fifth of al-Muharaam, the fifty-fourth month AH. I was absent for twelve nights, and arrived on the Sabbath, with seven days left in al-Muharram. …
It reached the Messenger of God that Sufyan B. Khalid b. Nubayh al-Huhali, the Lihyani, had alighted at Urana and its suburbs with people from his and other tribes: he had gathered a group to attack the Messenger of God.
Many people from the exhausted crowds had recourse to him. The Messenger of God called Abdullah b. Unays and sent him, by himself, on an expedition to kill Sufyan.
The Messenger of God said to him, ‘Attach yourself to Khuza’a.’ And Abdullah said, ‘O Messenger of God I do not know him. Describe him to me.’ The Messenger of God said, ‘Indeed when you see him you will dread him, fear him, and remember Satan.’ But I do not fear men, so I said, ‘O Messenger of God, I have not feared anything, ever.’
The Messenger of God said, ‘It will be a sign between you and him that you will shudder when you see him.’ … He said: I took my sword and nothing more, and set out to find the Khuza’a. I took the road until I reached Qudayd, where I found many Khuza’a who offered me beasts to ride on, and escorts, but I did not desire that.
I set out until I came to Batn Sarif, and then turned until I arrived at Urana. I began to tell those whom I met that I desired Sufyan b. Kahlid, for I would be with him, until I was in Batn Urana I met him marching, and behind him were the Ahabish and those who were attracted and drawn to him.
When I saw him I feared him, and recognized him by the description that the Messenger of God gave me. I saw myself drip with perspiration and I said, ‘By God, His Messenger said truly.’
It was the time of the Asar prayer when I saw him, and I prayed while walking and pointing with my head. When I was near him, he said, ‘Who is this man?’ I said, ‘A man from the Khuza’a. I heard that you were gathering against Muhammed, and I have come to be with you.’ He said, ‘Yes, indeed, I am in the group against him.’ …” 
Shaykh Allamah Shibli Nomani:
“SIRYA IBN UNAIS
Later on, in the same month, Sufyan Ibn Khalid who belonged to the tribe of Lahyan and was the chieftain of the mountainous areas of Uraina, planned an attack on Medina. The Prophet (p) sent, Abdullah Ibn Unais who by his diplomacy got an opportunity to kill Sufyan.” 
“THE MOBILIZATION OF ABDULLAH BIN UNAIS
On the fifth day, in the same month of Muharram, 4 A.H., the new news that Khalid bin Sufyan Al-Hudhali was gathering a force to raid the Muslim positions was received. The Prophet commanded Abdullah bin Unais to march out against the enemy and destroy them.
The Muslim force stayed away for eighteen days, during which time he successfully fulfilled his appointed task. He killed the leader of the rebels and brought his head back to Madinah on Saturday, seven before the end of Muharram. The Prophet gave him a staff as a reward while saying: ‘This will function as a sign of recognition for you and me on the Day of Resurrection.’ On his death bed, Abdullah requested that the staff be also place with him in his shroud.” 
 Kitab al-tabaqat al-kabir – Ibn Sa’d, volume 2, page 60
 Ibn Ishaq’s Sirat Rasul Allah – The Life of Muhammad Translated by A. Guillaume, page 666
 History of al-Tabari – The: The Last Years of the Prophet. volume 9, page 121
 The Life of Muhammad: Al-Waqidi’s Kitab Al-Maghazi [Translated: Rizwi Faizer] page 261
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – volume 2, page 77
 Sealed nectar – The Biography Of The Prophet, Safiur-Rahman Al-Mubarakpuri, page 297