A zen teaching (yulu) by Daoxin (580-651).
When mind of itself is peaceful and clean, then all that is needed is bold advance as if saving your head from burning. You must not slack off or get lazy. Try hard, try hard!
When beginners sit in meditation to contemplate mind: sit alone someplace. First straighten out your body and sit upright; let your robe be wide and your belt loose. Let your body relax: rub yourself down seven or eight times. Let the exhalations from the belly through the throat cease. Then you will find in abundance the purity, emptiness, and peace of inherent reality-nature.
When body and mind are properly attuned, when mind and spirit are at peace, then in deep mystic fusion, the breath is pure and cool. Slowly gather the mind until the path of the spirit is pure and sharp and the mindground is illumined and pure. As you perceive clearly and distinctly, inner and outer are empty and pure – this is the mind’s inherent nirvana. With this nirvana, the mind of the sages is manifest.
Though its real nature is formless, intent and proportion always remain. Thus, the profound luminous one never ends: it remains forever shining bright. This is called the buddha-nature, the enlightened real identity.
Those who see buddha-nature leave behind forever birth and death: they are called people who transcend the world. One who awakens to buddha-nature is called a bodhisattva.
Cleary, Jonathan C.: Zen dawn; early Zen texts from Tun Huang.
Boston 1986, p. 63