Pakiṇṇaka 32

The Buddha was staying at Sāvatthī. At that time, the bhikkhunīs built a toilet with a cover. Women brought dead fetuses, and put them inside. Later, outcasts, caṇḍālas, dug out the toilet. When they saw (the fetuses), they said: “These renunciates had abortions, and threw them in.” For this reason, the bhikkhunīs went and informed … The Buddha said: “From now on, a covered toilet is no longer allowed. You should build it with an open toilet bowl. If (a bhikkhunī) builds it with a cover, she breaks a minor vinaya rule. This is called the rule on toilets.