Reference for Chullin 52:3

לא שנו אלא שתמדו במים טהורים ונטמאו אבל טמאים מעיקרא לא

Raba remarked: This rule applies only if the Tamad was made with water that was clean and it subsequently became unclean, but not if the water was unclean from the outset.

Jastrow