Ansar (Arabic: الأنصار al-Anṣār, "The Helpers") is an Islamic term for the local inhabitants of Medina who took the Islamic Prophet Muhammad and his followers (the Muhajirun) into their homes when they escaped from Mecca (hijra).
They belonged to two main tribes of Azd, the Banu Khazraj and the Banu Aus. Azd is the same tribe that Ghamd tribe and Zahran tribe belong to.
Nusaybah bint Ka'ab, mother of Habib ibn Zayd
Ansar (Islam) Wikipedia
The Ansar helped Muhammad in several battles. One of the earliest battles they helped him in was the Invasion of Buwat. A month after the raid at al-Abwa that was ordered by Muhammad, he personally led two hundred men including Muhajirs and Ansars to Bawat, a place on the caravan route of the Quraysh merchants. A herd of fifteen hundred camels was proceeding, accompanied by one hundred riders under the leadership of Umayyah ibn Khalaf, a Quraysh. The purpose of the raid was to plunder this rich Quraysh caravan. No battle took place and the raid resulted in no booty. This was due the caravan taking an untrodden unknown route. Muhammad then went up to Dhat al-Saq, in the desert of al-Khabar. He prayed there and a mosque was built at the spot. This was the first raid where a few Ansars took part.
During the tenure of Caliphates after Muhammad, the Ansar mainly became important military elements in many conquests, (as indicated with the appointing of Thabit, bin Qays bin Shammas, an orator of Ansar), to lead Ansaris in support of Khalid ibn al-Walid in the Battle of Buzakha at the time of Caliph Abu Bakr. Later they also played a prominent role in the Battle of Yamama where Ansars under Al Bara bin Malik Al Ansari charged at a perilous moment of the battle marking its turning point. The battle of Yamama is also where the Ansar's most prominent warrior, Abu Dujana, fell.
In the year 24/645, during the caliphate of Uthman Ibn Affan, prominent Ansaris also held major positions like Al-Bara' ibn `Azib who was made governor of al-Ray (in Persia). He eventually retired to Kūfā and there he died in the year 71/690.
During the Umayyad era the Ansar became somewhat of an opposing political faction of the regime. They are described as closely affiliated with the Makhzum Clan Contingent rather than with the incumbent Umayyad. Such Ansar-Makhzum connections are described as forming a new elite local political hegemony in Hejaz.