al-'Aswad bin 'Amir Shadhan الأسود بن عامر شاذان · Hot Baths (Kitab Al-Hammam)

The Messenger of Allah (ﷺ) saw a man washing in a public place without a lower garment. So he mounted the pulpit, praised and extolled Allah and said: Allah is characterised by modesty and concealment. So when any of you washes, he should conceal himself.