Tractate Mikvaot - Chapter 1 - Mishnah 2

If one drew water from it in an unclean vessel and then a clean person drank [out of the pit], he becomes unclean. If one drew water [from it] in an unclean vessel and then drew water from it in a clean vessel, it becomes unclean. If one drew water [from it] in an unclean vessel and a loaf of terumah fell in: If he washed [his hands] in it, it becomes unclean; But if he did not wash [his hands] in it, it is clean.