Bûter, brea en griene tsiis: wa't dat net sizze ken, is gjin oprjochte Frys.
Translation: Butter, bread, and green cheese: whoever cannot say that is not a true Frisian.
Quoted in: The Linguist: Journal of the Institute of Linguists. Volumes 42-43, The Institute, 2003. p. 192
According to legend, Pier forced his captives to repeat this shibboleth to distinguish Frisians from Dutch and Low Germans.