What is one of the first recorded mentions of the sauna in Finnish literature?

Context: The sauna in Finland is an old phenomenon and its roots are difficult to trace, but its earliest versions are believed to be from 7000 BC. Bath houses were recorded in Europe during the same time period, but Finnish bathing habits were poorly documented for most of history. One of the first written mentions of what is believed to be the sauna customs of the forefathers of the Finns was written by Nestor the Chronicler in 1112. He told of "hot wooden saunas in which naked bathers beat themselves with branches and finally pour cold water over themselves".

During the Reformation in Scandinavia the popularity of saunas expanded to other countries because the European bath houses were being destroyed.

One reason the sauna culture has always flourished in Finland has been because of the versatility of the sauna. When people were moving, the first thing they did was to build a sauna. Finns have used the sauna to live in, eat, address matters of hygiene, and, most importantly, give birth in an almost sterile environment. Unlike many other, more densely populated places in Europe, the availability of wood needed to build and warm the sauna has never been an issue. Another reason for its popularity is that in such a cold climate, the sauna allows people warmth for at least a short period of time. However, it is just as popular in the summer as in the winter.

Answer: Nestor the Chronicler in 1112. He told of "hot wooden saunas in which naked bathers beat themselves with branches and finally pour cold water over themselves"".