Pakiṇṇaka 23

The Buddha was staying in Sāvatthī. At that time, Thullanandā bhikkhunī had big breasts. Just wearing her vest, she did walking meditation on the roof of the temple building. Lay people saw her from afar, and said to each other: “Look at this. It’s like bottle gourds floating on water.” For this reason, the bhikkhunīs went and informed … The Buddha said: “From now on, you should make a shoulder-covering robe. ‘A shoulder-covering robe’ means a cloth with pleats to spread over and cover the shoulders. If (a bhikkhunī) doesn’t make one, or doesn’t wear it, she breaks a minor vinaya rule. It’s not allowed for a bhikkhunī to do walking meditation on a high place while wearing only a one-layered vest. If she wears a one-layered vest in a secluded place, there’s no offense.” This is called the rule on vests.