Pakiṇṇaka 13

The Buddha was staying in Sāvatthī. At that time, Mahāpajāpatī went to the Buddha, paid respect with her head at his feet, and stood at one side. Then Mahāpajāpatī said to the Buddha: “World-honored One, a woman’s body stinks. May we be allowed to wash it?” The Buddha said: “You may wash it.” Then bhikkhunīs washed them outside, but the inside still stank. For this reason, she went and asked again. … She said: “May we wash inside?” The Buddha said: “You may wash. The method of washing is: One may to go in one finger joint. One may not go deeper. If (a bhikkhunī) washes deeper inside, to relieve a lustful mind, it’s a thullaccaya.” This is called the rule on washing.