The Uttara Tantra and Dzogchen
The earliest Buddhist text (350 A.D.) that defines rigpa as understood later in Dzogchen thought.
From the Uttara Tantra:
“The luminous nature of the mind is changeless, just like space. It is not defiled by adventitious stains, such as desire, born from false imagination.”
The author, Asanga, also mentioned a “samadhi of sunlight”, of which I can’t find further information.
Here is the entire tantra with a commentary by
Jamgön Kongtrül Lodrö Thayé “The Unassailable Lion’s Roar”
Khenpo Tsultrim Gyamtso Rinpoche http://bibleoteca.narod.ru/uttaratantra.pdf
Jamgön Kongtrul comments:
“Likewise the tathagatagarbha, the dharmadhatu which is clear light, abides all-pervasively within all beings as the nature of their minds. Yet this dharmadhatu is by nature completely pure. For this reason it never suffers the slightest pollution from the faults of beings, such as their mental poisons and so on.”
“This clear and luminous nature of mind is as changeless as space. It is not afflicted by desire and so on, the adventitious stains, which are sprung from incorrect thoughts. The nature of space is not changed through clouds, smoke, and so on. In the same way, the tathagatagarbha, the clear and luminous nature of the minds of all beings, is changeless. It is not in the slightest altered by the fact that the veils are puriﬁed or unpuriﬁed, and so on.”
“Its nature is absence of thought.” Although thought appears within it, yet it, itself, has no thoughts.