坐禪三昧經 2.54

2.54

Furthermore, having contemplated impurity and thus become disgusted with saṃsāra, one should contemplate the gateway of purity, fastening the mind on three places: the tip of the nose, between the eyebrows, and on the forehead. In these places, one should peel away an inch of skin, cleanse away the blood and flesh, and fasten the mind on the white bone, not allowing external thoughts; external thoughts of various objects are gathered and brought back to abide in the three focal points, constantly wrestling with the mind. It is like two people wrestling: if the practitioner wins, the mind yields; subduing it to make it stay is called one-pointedness of mind.