A 15th-Century Description
This passage is from the work of the Persian historian Mirkhwand, but it seems to rely on a now-lost Seljuk history called the Malik-nameh.
When [Seljuk] reached the region of Jand, God enlightened his heart with divine light. He sent a messenger to the governor of that province saying, ‘The reason of my coming to this place is so that I can join the path of the people of Islam. I beseech you to send a distinguished scholar of Islam to me so that by teaching the Qurʾan, the truths of the faith and Islam he may guide the erring pagans of the steppe to the wellsprings of true faith.’ His request was accepted and Seljuk and his followers and companions became Muslim.
A 13th-Century Description
This passage is from Bar Hebraeus, a Christian who wrote in Syriac in the thirteenth century.
[Seljuk] went forth from the land of Turan, that is to say Turks, to the land of Iran, that is to say Persians, under the pretense they were shepherds. And when they saw that Persia was flourishing with Islam, they took council together and said, ‘If we do not enter the Faith of the People in the country in which we desire [to live] and make a pact with them (or, conform to their customs), no man will cleave to us, and we shall be a small and solidary people.
Source, pp. 246-7.