Fríssbók or Codex Frisianus is an Icelandic manuscript written in the first quarter of the fourteenth century. It includes a copy of Magnússona saga (called Saga Sigurðar, Eysteins ok Ólafs in C. R. Unger’s edition). The whole manuscript has been digitized and is available at handrit.is!
Here is the part about skating, with the phrase “ek kvnna ok sva a isleggiom” (I could also go on skates) underlined.
The word for bone skates, isleggiom, crosses the line break. There are some abbreviations, as is normal for medieval manuscripts.
This is the only unambiguous reference to bone skates in the corpus of Old Norse literature, but not the only manuscript containing it: there are numerous manuscript copies of the saga.