By chance a party of the clan of Quraish were returning from Syria with some merchandise, which were passing from that way. Abdullah Ibn Jahsh and his men attacked them, killing Amr Ibn Al Hadrami, and capturing two others with some booty. .
On their return to Medina, Abdullah told the prophet of what had happened and presented the booty, the prophet reminded him that he had not been permitted to act as so, and refused to accept the booty, His companions were highly engaged and explained to Abdullah that he had something for which he had had no orders for, and fought during a prohibited month, which was not permitted. .
We have to know that Hadrami the man, who was killed, was an ally of Utbah Ibn Rabi"a, and Utbah was the chief of the community of Quraish.
Badr and many battles that followed were in consequence of revenge for this man's death (Hadrami), which is a general mistake, which lead to the beginning of the battle of Badr.
The Quraish clan had begun preparations to attack Medina, and they set up camp outside Mecca with 950 men, 100 horses, and 700 camels in contrast to the Muslims strength. The rich elders, Abu Jahl, Utabah and many more, each took turns to provide the daily fare of the army. This news did cause much anxiety in Medina. .
When Abu Sufyan was returning from Syria, he feared that the Muslims might intercept the trade caravans, as a result of his fears he sent a messenger well in advance to inform the leaders of Quraish of his fears. On receiving his message, a well equipped army of 1000 Meccans marched towards medina under the command of Abu Jahl with an ill equipped body of 313 people having between them, 2 horses and 70 camels only, the prophet went to Badr to meet the Meccan army. On approaching the wells of Badr, the prophet called a halt. Utbah Ibn Al Munthir objected and urged that they go forward and take possession of the wells, thus preventing water to the clan of Quraish; the prophet took his advice and ordered a change in the position of the army.