四分律比丘含注戒本 1.63

1.63

佛言：「是婬欲法也。男則二道，女則三道。若從道入道，從道入非道，從非道入道，若限齊，若盡入，乃至入如毛頭者，皆波羅夷。」

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."