If the body does not engage in killing, does not engage in stealing and does not engage in sexual misconduct, then the karma of the body becomes pure. If one does not lie, does not engage in frivolous speech, duplicitous speech, or in harsh speech, then the karma of the mouth becomes pure. If the mind does not engage in greed, hatred or stupidity then the karma of the intellectual mind becomes pure.
When in this manner the ten evil actions have become eternally cut off and the three karmic vehicles have become as pure as ice this brings about the essential [prerequisite] of the purified mind. Within this pure mind one develops an aversion for the suffering of the Saha world, brings forth the vow to go forth to rebirth, and peacefully nourishes one's establishment of the correct practice of mindfulness-of-the-buddha. Having done this, then the absolute essential requirement in mindfulness-of-the-buddha becomes the urgency of one's mind in relation to birth-and-death.
One first cuts off external conditions. One exclusively brings up the one thought. One takes the one phrase, “Amitabha” as the very root of one's life. It is not forgotten for even a single moment. It is not cut off for even the space of a single thought. During both the day and night, whether walking, standing, sitting or lying down, whether picking up a spoon or raising the chopsticks, whether twisting or turning around, whether bending down or raising up, whether moving or still, and whether at leisure or busy—in every single moment one refrains from any stupidity or mental darkness. One does not allow the intrusion of any other conditions.
If one uses the mind in this manner then after a time it becomes so pure and completely developed that one does not forget [one's mindfulness] even in one's dreams. Wakefulness and sleep become of a single suchness. When this becomes the case then one's skill becomes subtle and integrated and then becomes fused into a single, [continuous] entity. It is at this time that one gains realization of the power [of this practice].
If one's mindfulness reaches the state where one is single-minded and [one's thoughts are] not scattered, then when one approaches the end of one's life, the realm of the Pure Land will manifest before one. Then, one will naturally not be detained by birth and death. This being the case, one will then achieve as a response [the manifestation of] Amitabha who will emit light and lead one forth. This demonstrates the efficacy [of this practice's ability] to definitely bring about [the desired] rebirth. Thus this single-minded exclusive mindfulness assuredly is a correct mode of practice.
Dhyana Master Shrama.na Han-shan De-ching, The Record of Dream Wanderings