Reference for Niddah 116:8

לא עברה בשוק של טבחים אף בעליון אינה תולה

she may not do so even though [the blood was found] on the uppermost shirt. How so? If she passed through a butchers' market she may attribute the blood to it even though it was found on the lowest shirt, but if she did not pass through a butchers' market she may not attribute the blood to it even if it was found on the uppermost.

Jastrow