Page "Gevil" Paragraph 6

Maimonides wrote that it is the law transmitted to Moses on Mount Sinai that a Torah scroll must be written on gevil or alternatively on klaf and that scrolls written on an alternative material are invalid for use, however it is preferable that they be written on gevil ( Maimonides, Hilkhoth Tefillin 1: 14 ).

According to Maimonides, this law is a requirement for all kosher Torah scrolls.