Amr bin Umayya (r.a.)
One of the most important incidents that Amr bin Umayya, one of prominent Companions of the Prophets, experienced was the Incident of Bi'r Mauna, which resulted in the martyrdom of 39 Companions. The tribe of Sons of Nadr asked the Messenger of Allah to send them some people to teach them Islam. Thereupon, the Messenger of Allah sent a group of 40 Companions with the leader of Sons of Nadr. When the group arrived at a place called "Bi'r Mauna", they were surrounded by Sons of Nadr. The Companions realized that it was a trap, but it was too late. The polytheists killed the 39 Companions ruthlessly. At that time, Amr bin Umayya and a friend of his were grazing their camels in a place nearby. When they saw birds flying over Bi'r Mauna, they suspected something. While they were going there, a woman who saw the incident informed them about it. They rushed to the place but they were late. They started to fight the polytheists. Amr bin Umayya was captured but his friend was martyred. After a while, Amr bin Umayya escaped; while returning to Madinah, he met two people on the way; he found out that they were from Sons of Nadr. He could not forget what they had done to the Companions and to him. He followed them secretly. When the two men fell asleep under a tree, he killed both of them. He then returned to Madinah and entered into the presence of the Messenger of Allah. The Messenger of Allah was saddened a lot when he heard what happened to the Companions. He cursed Sons of Nadr a lot. When Amr bin Umayya told the Messenger of Allah that he had killed two people from Sons of Nadr in order to get the revenge of the Companions, the Messenger of Allah became sad and said,
"What a terrible thing you did! You killed two people who were given the guarantee of safety and protection by me. I will pay blood money for them."
Afterwards, the Messenger of Allah paid the blood money for those two people. He was not glad when he heard that two of them had been killed though they had killed so many Muslims. On the contrary, he was angry and showed a real example of justice by paying blood money for them. However, Sons of Nadr began to bear the consequences of what they had done after a while. A great famine occurred in this tribe, who were cursed by the Messenger of Allah. In addition, a severe fever epidemic began to spread. 700 people died of the fever epidemic only. 
Usdul-Ghaba, 4: 86; Tabaqat, 1: 259; 2: 93-94, Tabari, 3: 32.