Mahāyāna Mahāparinirvāṇa Sūtra
The doctrine of "Tathāgata embryo" or "Tathāgata womb (Tathāgatagarbha)," also known as "Buddha nature, matrix" or "principle" (Skt: "Buddha-dhātu") is important in all modern Mahāyāna traditions, though it is interpreted in many different ways. Broadly speaking, Buddha-nature is concerned with explaining what allows sentient beings to become Buddhas. The earliest sources for this idea may include the "Tathāgatagarbha Sūtra" and the "Mahāyāna Mahāparinirvāṇa Sūtra". The "Mahāyāna Mahāparinirvāṇa" refers to "a sacred nature that is the basis for [beings] becoming buddhas", and it also describes it as the 'Self' ("atman").