四分律比丘戒本 1.p128

1.p128

A monk in good health ought to bathe twice a month; if he does so more frequently, except on allowed occasions, it is pācittiya. The occasions are, during the hot season, during sickness, at the time of work, when there is much wind (and therefore dust), and when there is much rain (and therefore mud), and also after a long journey. This is the Rule.