Mishnah — Seder Nezikin, Tractate SanhedrinChapter 1…
“ Levirate nullification - When a man dies and leaves a widow but he has not yet born children, the responsibility to bear heirs falls to the nearest of kin to the man. [This is called Levirate marriage.] If the man and widow choose not to fulfill this responsibility, he may take off his sandal and the widow spits in the sandal. ”
