I found this passage from the fourth patriarch Doshin Daii Zenji (580-651) particularly remarkable as it echos almost exactly the instructions I received nearly 1400 years later.
It is from Doshin’s “Instructions for Novices” in Masters of the Lanka, translated by Sam Van Schaik in his book “The Spirit of Zen”.
“To begin, sit with your body upright, in comfortable clothes without a belt. Relax your body and loosen your arms and legs by rubbing them seven or eight times. Allow your mind to come to rest in your abdomen, and let your breath out completely. [from the footnotes: in some versions of the text the word xin or “mind” is missing and this passage can be translated “force all the air out of your abdomen.”] You will suddenly realize your nature to be pure and lucid, calm and clear, with body and mind in harmony.
Then as you pacify mind and spirit, subtle and profound, with calm, refreshing breathing, gradually turn the mind within. The spiritual path will become clear and sharp, the mind’s ground pure and luminous. When you examine this luminosity, you find both internal and external are empty and pure. This is mind’s natural stillness.
This stillness is the manifestation of the Buddha’s mind itself. Though it is formless in nature, it always has purity of intention. This spiritual energy is never exhausted; it is always present in its bright clarity. This is what we call the Buddha nature.”