Abu Hurairah (may Allah be pleased with him) reports that the Messenger of Allah (peace be upon him) said: "When a dog drinks from the vessel of any one of you, he should pour out that (liquid), and he should wash that vessel seven times. And when the strap of any one of your (shoes) breaks, he should not walk in that (shoe) until he has it repaired."
This contains evidence against the position of those scholars who claim that water is pure and washing the vessel is a devotional act, because it is impossible that the Prophet (peace be upon him) would command to pour away (and waste) pure, non-impure water.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب ذكر الماء الذي لا ينجس، والذي ينجس إذا خالطته نجاسة / 98