Which preposition to use with shibboleth

of Occurrences 15%

For two centuries the mosques and the mint proclaimed the shibboleth of the Shiahs.

to Occurrences 1%

"Alas!" says Fuller, it is a shibboleth to a child's tongue wherein there is a confluence of hard consonants together; and then he continues, "What the child could not pronounce the parents do not practise.

Which preposition to use with  shibboleth