We will discuss the measurement of the kurr, if Allah سبحانه وتعالى, the Exalted, wills. Then he (Shaykh Al-Mufid) said: "If he places his hand into the water without washing it as we described, it does not invalidate the water nor affect its suitability for purification." This has already been discussed earlier. Then he (Shaykh Al-Mufid) said: "If he places his hand into the water while it carries impurity, it renders the water impure if it is stagnant and a small quantity. In such a case, purification with it is not permissible." What indicates this is (following hadith):
Commentary #27