These narrations collectively indicate that running water does not retain impurity in a legal sense.
Then the Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: "There is no requirement for istinja (washing the private parts) for those purifying themselves from the impurities of sleep or passing wind; it is required only for one who has defecated." This is supported by the principle that obligations cannot be placed upon individuals without definitive evidence. Since there is no explicit evidence in Islamic law mandating istinja for sleep or passing wind, there is no obligation for it. Additionally, it is supported by (following hadith):
Commentary #35