The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: "If water is contained in a limited vessel and an impurity falls into it, it cannot be used for Wudu', and it must be discarded." This is supported by what we have previously mentioned: when water is less than a kurr (large volume), it becomes impure if impurities enter it. Once its impurity is established, it cannot be used under any circumstances, and this ruling is undisputed. Additional evidence also supports this conclusion.
Commentary #19