四分律比丘含注戒本 1.126

1.126

實心故作出不淨意。前境有六，若於內色、外色、內外色，若水，若風，若空，隨作方便，若出，即犯。

With actual intent, he deliberately gives rise to the thought of emitting impurity. There are six external objects: whether toward internal form, external form, internal-and-external form, or water, or wind, or empty space, whenever he applies means, if emission occurs, he immediately commits an offense.