I want to know about the authenticity of the following narration in Ibn Ishaq's biography:-
When God wished to display His religion openly and to glorify His prophet and to fulfil His promise to him, the time came when he met a number of the Helpers at one of the fairs; and while he was offering himself to the Arab tribes as was his wont he met at al-'Aqaba a number of the Khazraj whom God intended to benefit. 'Asim b. 'Umar b. Qatada told me on the authority of some of the shaykhs of his tribe that they said that when the apostle met them he learned by inquiry that they were of the Khazraj and allies of the Jews. He invited them to sit with him and expounded to them Islam and recited the Quran to them. Now God had prepared the way for Islam in that they lived side by side with the Jews who were people of the scriptures and knowledge, while they themselves were polytheists and idolaters. They had raided them in their district and whenever bad feeling arose the Jews used to say to them, ‘A prophet will be sent soon. His day is at hand. We shall follow him and kill you by his aid as 'Ad and Iram perished.’ So when they heard the apostle's message they said one to another: ‘This is the very prophet of whom the Jews warned us. Don't let them get to him before us!’ Thereupon they accepted his teaching and became Muslims, saying, ‘We have left our people, for no tribe is so divided by hatred and rancour as they. Perhaps God will unite them through you. So let us go to them and invite them to this religion of yours; and if God unites them in it, then no man will be mightier than you’. Thus saying they returned to Medina as believers. ... When they came to Medina they told their people about the apostle and invited them to accept Islam until it became so well known among them that there was no home belonging to the Helpers but Islam and the apostle had been mentioned therein. (Ibn Ishaq's Sirat Rasul Allah, page no. 197-198)
What is the authenticity of this narration in Ibn Ishaq?