Pakiṇṇaka 9

The Buddha was staying in Sāvatthī. At that time, there was a young bhikkhunī who was attractive. Her breasts were exposed, and when people saw it, they all laughed at her. The bhikkhunīs for this reason went and informed the World-honored One. … She answered: “I really did this.” The Buddha said: “From now on, you should make a vest. The method to make it’s as explained above. She should first put on a breast covering cloth, and over that wear her other robes. If she doesn’t keep a vest, she breaks a minor vinaya rule. If she has one, but doesn’t wear it, she also breaks a minor vinaya rule.” This is called the rule on vests.