Pakiṇṇaka 18

The Buddha was staying in Sāvatthī. At that time, a lustful bhikkhunī flushed her vagina in a waterfall, and emitted sexual fluids. Then doubt and remorse arose in her mind. The bhikkhunīs told Mahāpajāpatī, and she went and informed the World-honored One. The Buddha said: “From now on, it’s no longer allowed to flush the vagina in a waterfall. ‘A waterfall’ means water falling downwards. If a bhikkhunī bathes in a waterfall, she should hold a cloth item for protection. If she flushes her vagina in a waterfall, or with water falling from the roof of a building, to relieve a lustful mind, it’s a thullaccaya. If she washes in a waterfall, or with water falling from the roof of a building, she may not turn the body towards the water. It should fall on the back. If she turns the body towards the water to relieve a lustful mind, it’s a thullaccaya.” This is called the rule on waterfalls.