Pakiṇṇaka 14

The Buddha was staying in Sāvatthī. At that time, bhikkhunīs who had their menses soiled their beds and mats. Mahāpajāpatī went to the Buddha and said to him: “World-honored One, may we make a cloth for the impurity of the menses?” The Buddha said: “You may. You should use an old cloth to make it. You may not make it with a hard item. You may also not insert it deeply to create thoughts of sexual desire. It should be made with a soft item to block the vagina. If (a bhikkhunī) uses a hard item, or inserts it deeply, to relieve a lustful mind, it’s a thullaccaya.” This is called the rule on the menstrual cloth.