The history of Buddhism and the initial period of its spread in Transbaikalia in the XVII-XVIII centuries are insufficiently studied due to the lack of archival materials and sources, while the surviving historical records provide different information about some events, such as the year of construction of the first Buddhist temple and the first sermon, the years of D.-D. Zayaev's life. The earliest extant sources are "Autobiography" and "Travel Notes" written by D.-D. Zayaev in 1768. They contain valuable information not only about the history of Buryat Buddhism, but also about the life and activities of D.-D. Zayaev (1711-1776), the first Buryat pandita Khambo-Lama, whose name is associated with the beginning of the spread of Buddhism among the Buryats and its institutionalization in the Russian Empire, but also with the description of his pilgrimage to Tibet. In this regard, in order to clarify some dates on the early history of Buryat Buddhism, texts related to him have been searched for and analyzed in the funds of the Center of Oriental Manuscripts and Xylographs (CVRK) of the Institute for Mongolian, Buddhist and Tibetan Studies of the Siberian Branch of the RAS. It was established that of the six sources found, the most informative work is "The Legend of the Tsongol Kin Pandita Khambo-Lama Zayaev's Journey to Tibet" in the Old Mongolian script, stored in the Mongolian collection of the CVRK. As a result of the study, it was revealed that this work is a late manuscript consisting of the "Travel Notes", the Biography of D.-D. Zayaev and of an anonymous manuscript containing a brief description of Buddhist sacred places and shrines in India, Nepal, Tibet and Mongolia, what has determined its structure consisting of 3 parts. It was found out that this manuscript contains information about the socio-economic situation of the population of Tibet and neighboring countries, as well as information about the history of Buddhism in Transbaikal region in the middle of the XVIII century. It testifies to Russia's geopolitical interests in this region of Inner Asia and the peoples living there, as the author reports, saying that it was written at the behest of Catherine II, who outlined a range of issues: "to describe what I’ve seen on my way to Tibet, the length of the road and distance, as well as about the customs and habits of people". The unique historical information preserved by the Buryat pilgrim and the Russian citizen D.-D. Zayaev is also relevant for conducting comparative studies of the phenomenon of modern pilgrimage that is becoming popular not only for believers but among ordinary lay population.