Please answer a question about the following article about Estonia:

The Oeselians or Osilians (Estonian saarlased; singular: saarlane) were a historical subdivision of Estonians inhabiting Saaremaa (Danish: sel; German: Osel; Swedish: Osel), an Estonian island in the Baltic Sea. They were first mentioned as early as the second century BC in Ptolemy's Geography III. The Oeselians were known in the Old Norse Icelandic Sagas and in Heimskringla as Vikingr fra Esthland (Estonian Vikings). Their sailing vessels were called pirate ships by Henry of Latvia in his Latin chronicles written at the beginning of the 13th century.

Where was the initial mention of the Oeselians?
Ptolemy's Geography III