In 632 A.D. the Prophet (S.A.W) deputed Ali (A.S.) to preach Islam in Yemen. Khalid Bin Waleed had laboured for six months to bring the Yemenites to Islam but had miserably failed, because the people of Yemen were proud of their heritage and culture and took the greatest pride in their traditions and institutions. The Jewish and Christian colonies in and around Yemen were also famous for their soldiers who were well versed with their scriptures and spoke with authority in the matter of their religion.

Before Ali (A.S.) left for Yemen, the Prophet (S.A.W) examined the three hundred people Ali (A.S.) was taking with him. He gave a black flag to Ali (A.S.) and tying a black turban on his head said, "Proceed to Yemen and preach its inhabitants the divine law of Islam. You have to enlighten them with the commandments of Allah as revealed in the Quran. Teach them the methods of prayers and extend them the invitation to accept Islam. I swear by Allah that one conversion by you is far nobler and better than all the wonders of nature on
which the sun rises and sets."

On reaching Yemen, Ali (A.S.) read the Prophet (S.A.W)’s letter to the people and spoke to them the doctrine of Islam in such a way that the entire Hamadani tribe embraced Islam on the very first day. The Christian priests and the Jewish Rabies held daily discussions with him on religious doctrines and dogmas and Ali (A.S.)’s sermons on the unity of Allah and the Islamic expositions of the values of life greatly impressed them. After one of his sermons the Christian priest Kaab a distinguished scholar and critic embraced Islam and others soon followed him. Similarly many Jewish Rabies and other tribes of Yemen embraced Islam.