The Battle of Ayn al-Tamr (Arabic: ) took place in modern-day Iraq (Mesopotamia) between the early Muslim Arab forces and the Sassanians along with their Arab Christian auxiliary forces. Ayn al-Tamr is located west of Anbar and was a frontier post which had been established to aid the Sassanids.
The Muslims under Khalid ibn al-Walid's command soundly defeated the Sassanian auxiliary force, which included large numbers of non-Muslim Arabs who broke earlier covenants with the Muslims. According to non-Muslim sources, Khalid ibn al-Walid captured the Arab Christian commander, Aqqa ibn Qays ibn Bashir, with his own hands.
After the battle, some Persians had hoped that the Muslim commander, Khalid ibn al-Walid, would be "like those Arabs who would raid [and withdraw]." However, Khalid continued to press further against the Persians and their allies in the subsequent Battle of Dawmat al-Jandal.
When the Muslim army conquered the town of Ayn al-Tamr they found a number of Arab Christian priests in a monastery. One of them was called Nusair another called Sirin. They both embraced Islam. Nusair was the father of Musa bin Nusayr, the supreme commander of the forces which later conquered Spain under the leadership of Tariq bin Ziyad, the second in command for Musa bin Nusayr. Sirin, the other convert, was the father of the scholar Ibn Sirin who became one of the more celebrated Muslim theologians.