The light of Islam among the Ansar comes to fruitfulness after the call and invitation of the Prophet Muhammed (PBUH).
Though the Ansar were long time allies of the Jews, and whenever they had a quarrel, the Jews will humiliate them, saying that a Prophet is coming and they will accept him, they will often say ‘we shall kill you similar to the killing of Aad and Iram’
So when Prophet Muhammed (PBUH) invited the Ansar to Allah, they said among themselves that this is similar to the Prophet the Jews had been talking about, and they determine to pledge alliance to the Prophet before the Jews. Therefore, the Ansar accept Islam.
That’s how the dawn of Islam started among the Ansar. That said, let look at various sayings, including hadith about how Islam was among the Ansar.
Ibn Ishaq related: When Allah wanted to manifest His religion, empower His Prophet, peace and blessings of Allah be upon him, fulfil His promise to him, Allah’s Messenger, peace and blessings of Allah be upon him, used to go out in the Hajj season and would meet the groups of the Ansar. Again he would go to offer himself to the Arab tribes as he would do every year. When he was at Al-‘Aqabah, he met a group of the Khazraj for whom Allah intended good.
Asim Ibn Umar Ibn Qatadah narrated to me from some elders of his people. They said: When Allah’s Messenger, peace and blessings of Allah be upon him, met them, he said: “Who are you?” we said, “A group of the Khazraj.” He asked again: “From the allies of the Jews?” we replied in the affirmation. He said: “Would you like to sit for a while so that I may speak with you?”
They sat and he invited them to Allah, presented Islam to them and recited the Qur’an to them. Allah had paved the way for Islam in that they were living adjacent to the Jews who were from the People of the Book and had knowledge while they were from the polytheists and idol worshippers.
The Jews had been suppressed in their land.
However, anytime there was a dispute between them, the Jews would say: “A Prophet will be raised and his coming is high. When he comes, we shall pledge allegiance to him and along with him, we shall kill you similar to the killing of Aad and Iram.”
When Allah’s Messenger, peace and blessings of Allah be upon him, spoke to that group and invited them to believe in Allah, they said to one another: “O people, beware, by Allah, this is the same Prophet that the Jew has promised you so do not let them precede you in accepting him.”
Thus, they responded positively to what they were invited to.
They accepted him and believed in the truthfulness of Islam. They said to him: “We have left our people back at home and there are no people who have a greater enmity and evil running between them than we. Perhaps, Allah shall unite them through you.
We shall approach them and invite them to your matter. We shall also present and invite them to the religion which we have accepted. If Allah unites them through you, no man will be more powerful than you.” Thereafter, they left and returned to their country after they had believed and accepted the truth of Islam.
So Prophet Muhammed (PBUH) invited the Ansar to Allah, as a result, they pledge alliances and accept Islam, even before the Jews. And that is how the light of Islam started among the Ansar.