The lDe'u chronicles
Two related texts, written in the thirteenth century, contain substantial sections describing the legislative and administrative activities of Songtsen Gampo. They are generally referred to as:
Chos ’byung chen po bstan pa’i rgyal mtshan of lDe’u Jo sras (Jo sras)
rGya Bod kyi chos ’byung rgyas pa of mKhas pa lDe’u (lDe’u)
Jo sras is the briefer text and is quite possibly earlier, while lDe’u is more detailed. Both probably date from the mid to late thirteenth century.
Both texts are in two parts. The first is a long exposition of Indian Buddhism, presented as a commentary on a (non-extant) verse text. The second is a history of Buddhism in Tibet, presented as a genealogy of the Tibetan royal families. Within the second part is a long section, sometimes known as The Section on Law and the State, which describes the legislative and administrative activities of Songtsen Gampo.