Pakiṇṇaka 19

The Buddha was staying at Sāvatthī. At that time, a bhikkhunī bathed in rapidly flowing water. Lust arose in her mind, and she went against the current, and emitted sexual fluids. For this reason, the bhikkhunīs went and informed … “From now on, it’s no longer allowed to go against the current in rapidly flowing water, and have it touch the vagina. ‘Flowing water’ means a mountain stream, or rapidly flowing water. If she turns towards the current, and goes against the water to relieve a lustful mind, it’s a thullaccaya. If she washes in rapidly flowing water, she may not turn towards the current. She should turn the back towards it. If she turns towards the current, she breaks a minor vinaya rule.” This is called the rule on flowing water.