Some time in A.D. 628, news was brought to the Holy Prophet that the Banu Al-Mustaliq were gathering together against the Muslims. The Holy Prophet decided that before the enemy could gather strength the Muslims should fall upon and break their power.
The Holy Prophet led the campaign personally. The Muslim force that accompanied the Holy Prophet included Umar. It also included Abdullah bin Ubayy, an Ansar chief whose loyalties to Islam and the Holy Prophet were doubtful.
A confrontation with the Banu al-Mustaliq took place at Muraysi on the road to the sea. The Banu-Mustaliq were defeated with heavy losses, and the Muslims captured considerable booty including many captives both men and women.
After the Banu al-Mustaliq were overpowered and put to flight, the Muslim force halted at a watering place. Here Jahja bin Masud a servant of Umar led his horse to the watering place. There a quarrel arose between him and Sinan b. Wabar al-Juhani, an Ansari. In the quarrel Sinan called the men of the Ansar to his aid, while Jabja invoked the assistance of the Muhajreen.
The incident was exploited by Abdullah b. Ubayy, and he spoke unbecoming words against the Muhajreen. He said:
"The Muhajreen dispute our priority. They want to overpower us in our own city. Nothing fits us and the vagabonds of Quraish as the ancient saying 'Feed a dog and it will devour you'."
Zaid b. Arqam an Ansar boy who heard Abdullah bin Ubayy indulge in such talk reported to the Holy Prophet what he had said. Umar who was sitting with the Holy Prophet asked for the Holy Prophet's permission to kill Abdullah bin Ubayy. The Holy Prophet said:
"Umar be calm and patient. If I permit you to kill Abdullah, will not the people say that Muhammad kills his own followers?"
The Holy Prophet immediately ordered a march to Madina. It was noon, an unusual time to travel, but all concerned complied with the order thinking that something unusual had happened and the Prophet of Allah knew what was best.
Abdullah bin Ubayy saw the Holy Prophet, and denied having said what had been reported against him. There were others even among the Ansars who held that Abdullah bin Ubayy had said what he should not have said. The Holy Prophet, however, chose to be silent.
When the party reached Madina, the unfortunate episode became the talk of the town. According to the consensus of opinion, Abdullah b. Ubayy was to blamed. When the son of Abdullah b. Ubayy came to know of what his father had said, he waited on the Holy Prophet and said:
"I have heard that you want to kill my father for what you have heard about him. If you intend to kill him, then order me to do so, and I will bring you his head. I am afraid that if you order some one else to kill him, my soul will not permit me to see his murderer walking among men, and I shall kill him thus killing a believer for an unbeliever and so I would go to hell."
The Holy Prophet said, "Thank God there are persons among the Muslims who would be prepared to kill their fathers for the sake of Islam."
Turning to the son of Abdullah b. Ubayy the Holy Prophet said:
"Thank you for your offer. I do not contemplate any such action against your father."
The son of Abdullah praised the Holy Prophet for his magnanimity. After he had left, the Holy Prophet turned to Umar and said:
"Now Umar what do you think. Had I allowed you to kill him on the day you wanted me to permit you, his people would have risen in his support. Today you see, his own son is prepared to kill him." Thereupon Umar replied: "Verily I know not what you know. Your vision is certainly penetrating and blessed."