Imam al-Shafi'i (may Allah have mercy on him) said: If someone says, "From where (do you know) that it did not become impure?" it will be said to him: the Messenger of Allah (peace and blessings be upon him) performed ablution, and there is no doubt that some of the ablution water fell on his clothes, and it is not known that he washed his clothes because of it, nor that he changed them, and I do not know of anyone among the Muslims who did that. It is understood that when no impurity touches water, it does not become impure.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1121