Fleeing persecution and finding a sanctuary in Medina, the Muslims might have thought it to be the end of the persecution and oppression they sustained at the hands of the polytheistic Quraysh in Mecca. But, far from it, the Quraysh became more determined to attack and kill the Muslims down there at Medina. Abdur-Rahman ibn Ka’b ibn Malik reported that the polytheistic leaders of Mecca wrote letters to Abdullah ibn Ubay and other leaders of Medina. That if they wouldn’t drive the Muslims out of Medina, they would come and slaughter everyone there. This incident occurred before the battle of Badr. The reports state:
“AbdurRahman ibn Ka’b ibn Malik reported on the authority of a man from among the companions of the Prophet (p): The infidels of the Quraysh wrote (a letter) to Ibn Ubayy and to those who worshipped idols from al-Aws and al-Khazraj, while the Apostle of Allah (p) was at that time at Medina before the battle of Badr.
(They wrote): You gave protection to our companion. We swear by Allah, you should fight him or expel him, or we shall come to you in full force, until we kill your fighters and appropriate your women. When this (news) reached Abdullah ibn Ubayy and those who were worshippers of idols, with him they gathered together to fight the Apostle of Allah (p). When this news reached the Apostle of Allah (p), he visited them and said:
The threat of the Quraysh to you has reached its end. They cannot contrive a plot against you, greater than what you yourselves intended to harm you. Are you willing to fight your sons and brethren? When they heard this from the Prophet (p), they scattered. …” (Abu Dawud Book 19, Hadith 2998)
Kitab al-Maghazi – Ma’mar ibn Rashid:
“The disbelievers of Quraysh wrote to Abd Allah ibn Ubayy ibn Salul and also to those members of the Aws and Khazraj tribes who were idolaters. This occurred while the Messenger of God resided in Medina but before the incident at Badr. The disbelievers said: ‘You have given shelter to our tribesman, and you remain the more numerous of Medina’s inhabitants. We swear by God that you had better either kill him or expel him, or else we will rally the Arabs to help us and march against you in our full numbers, slaying your warriors and ravishing your women!’
When word of this reached ibn Ubayy and the idolaters who were with them, they exchanged messages and convened. They dispatched a message to the Quraysh, agreeing to murder the Prophet and his companions. When the Prophet caught wind of this, he and a band of his men confronted them, saying: ‘The threats of the Quraysh have certainly wreaked havoc upon you.
They didn’t beguile you nearly as much as you wish to beguile yourselves. You are the ones who seek to kill your own sons and brothers.’ When they heard the words the Prophet had spoken, they dispersed and went their separate ways, and words of these matters reached the infidel Quraysh.” (Kitab al-Maghazi, Ma’mar Ibn Rashid, page 67) 
From the above early canonical reports, we are told that once they finished reading the letter that were sent by the Quraysh to the leaders of Medina, Abdullah ibn Ubai, and his companions gathered together all their men in order to wage war against Muhammad (p) and his people. Notice, Muhammad (p) and his people just left Mecca, fleeing persecution, yet here again the Quraysh followed them and inciting the Medinans against them!
The Quraysh pursued them down there for more persecution just for the Muslims changing their former faith from polytheism to Islam.
When the news reached the Muslims that Abdullah ibn Ubai, and his men were preparing to kill the Muslims based on Quraysh instigations, the Prophet (p) went over to them and told them that the Quraysh threatening them had an implication, that what they (the pagan Quraysh) had planned for them was far worse than what they planned against the Muslims:
“The threat of the Quraysh to you has reached its end. They cannot contrive a plot against you, greater than what you yourselves intended to harm you. Are you willing to fight your sons and brethren? When they heard this from the Prophet (p), they scattered.” (Abu Dawud Book 19, Hadith 2998)
When Abdullah ibn Ubai and his men heard this profound speech made by Prophet Muhammad (p), they all went away and abandoned the plan of fighting the Muslims.
In this we see a marvellous example of what a great leader Prophet Muhammad (p) (p) was. He was able in a short speech to bring an end to a civil war happening in the Madinan community. After the Quraysh had clearly declared war on the Muslim community in Medina, the Muslims were given permission to fight against the Quraysh.
 The Expeditions (‘Kitab al-Maghazi’) – An Early Biography Of Muhammad By Ma’mar Ibn Rashid [Translated: Sean W. Anthony – Copyright 2014 by New York University], page 67