四分律比丘含注戒本 1.63

1.63

The Buddha said: "This is the dharma of sexual intercourse. For a man there are two tracts; for a woman there are three tracts. Whether entering a tract from a tract, entering a non-tract from a tract, or entering a tract from a non-tract, whether limited to an entrance, fully entered, or entered even as much as the tip of a hair, all are pārājika."