When any of you performs ablution, let him sniff water in his nose and then blow it out. Whoever cleans his private parts with stones, let him do so with an odd number of stones

The Prophet (may Allah's peace and blessings be upon him) clarified some of the Tahārah (ritual purity) rulings including the following: First: Whoever performs ablution must sniff water into the nose by inhaling and then blow it out by exhaling.

Second: Whoever wants to clean his private parts from impurities with something other than water, like stones and the like, should do so with an odd number of stones; the least of which is three, and the most of which is what is necessary to fully cleanse the impurities off his body. Third: Whoever wakes up from night sleep should not put his hand in the water vessel to perform ablution until he washes it three times outside the vessel, for he does not know where his hand was during his sleep.

It might have been affected by some impurity or the devil might have been meddling with it carrying to it what could either be harmful to man or what could make the water impure.