Taixi jing: Classic of Embryonic Breathing
The Celestial Lord Jade Emperor said:
The embryo is produced from the qi within. The qi has the embryo within the breath. When the qi enters the body, this is life. When the shen leaves the body, this is death. Knowing shen and qi, longevity is possible. So do not depart from nourishing shen and qi.
When the shen moves, the qi moves. When the shen abides, the qi abides. If you desire longevity, then shen and qi should mutually abide.
When the mind is without random thoughts, without coming and without going, without exiting and without entering, then it naturally and constantly abides.
Striving in this practice, is the true path of the Dao!