Letter from Buddhaguhya
Buddhaguhya was an Indian master, known in Tibetan as Sangs rgyas gsang ba. Based in Varanasi, he was invited by Tri Song Detsen (Khri srong lde'u btsan) to go to Tibet, but was unable to do so, so he wrote the king a letter, known as the Bhoṭasvāmidāsalekha. This letter is addressed to the king, his people, and the Tibetan clerics, with instructions about how to behave and descriptions of ideal models for the king, ministers, and different levels of monks.
The author praises Tri Song Detsen for having introduced the laws of the ten virtues into Tibet and pacified the realm.