Chronica Albrici Monachi Trium Fontium
Chronica Albrici Monachi Trium Fontium (1232-1241)
The conventional dating of the Prester John letter to c. 1165 comes largely from Albéric and his chronicle. Albéric also comments on Alexander III's Letter to Prester John.
From Silverberg:
"In the chronicle of Albéric of Trois Fontaines, written between 1232 and 1252, one finds this entry for the year 1165:
At this time Presbyter Ioannes the King of the Indians sent his letters full of marvelous things to different kings of Christendom, but especially to Manuel the Constantinopolitan Emperor, and to Frederick, the Emperor of the Romans." (pg. 40)
To read the English translation of the relevant excerpts, see Brewer, pp. 146-149.