Tathāgata-garbha Sūtra 

‘The Sūtra of the Embryonic Buddha’, the foundational source for the tathāgata-garbha theory. It is a short text which demonstrates the existence of an inherent potentiality for enlightenment (bodhi) in all beings through a series of similes showing how this potentiality or Buddha-nature lies concealed within each individual. Apart from some Sanskrit fragments, the text only survives in Tibetan and Chinese translations.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)