Pakiṇṇaka 33

The Buddha was staying in Sāvatthī. At that time, women from Sakyan and Mallan families bathed in bathrooms. Then young men entered there, and raped them. The bhikkhunīs told Mahāpajāpatī … “From now on, it’s no longer allowed to enter a bathroom. If (a bhikkhunī) is sick, she may light a fire in a room, apply oil, and have a massage. If a bhikkhunī enters a bathroom to bathe, she breaks a minor vinaya rule. This is called the rule on bathrooms.