hebrew - Source for Osnat's Locket

I've heard that the reason Joseph knew he could marry Osnat was because he saw her wearing a locket / pendant with a Hebrew inscription on it so he knew she was a Hebrew. Has anyone heard this story and what is it's source?

Answer

פרקי דרבי אליעזר לז

See here: http://he.wikisource.org/wiki/%D7%A4%D7%A8%D7%A7%D7%99_%D7%93%D7%A8%D7%91%D7%99_%D7%90%D7%9C%D7%99%D7%A2%D7%96%D7%A8_%D7%A4%D7%A8%D7%A7_%D7%9C%D7%97

And a slightly different version cited by R' Kasher: http://hebrewbooks.org/pdfpager.aspx?req=51482&st=&pgnum=168