On the Arrival of the Partriarch of the Indians to Rome under Pope Calixtus II
Although de Adventu does not invoke the name "Prester John" directly, its linking of a rich Christian patriarch of India and the figure of St. Thomas allows scholars to see it as one of the early, potentially direct influences on the Letter of Prester John.
On the Miracles of St Thomas:
From Slessarev, Prester John: The Letter and the Legend:
The city over which he ruled… was the capital of India and its name was Hulna. In circumference it extended for four days’ journey and its walls were so thick that two Roman chariots set abreast could be driven on them. Through the middle of the city flowed the River Physon on its course from the earthly paradise. Its waters were crystal clear and they were full of gold and precious stones. Hulna’s population consisted exclusively of Christians among whom there were no heretics or unbelievers, because such persons either came to their senses or died (11).
From Silverberg, The Realm of Prester John:
More on St. Thomas and medieval Christianity in India.