Dog in Islam
Abu Hurairah (may Allah be pleased with him) reported that the Prophet (peace and blessings of Allah be upon him) said: "If a dog drinks from one of your vessels, let him wash it seven times." Muslim added: "The first time, with earth." Abdullah ibn Mughaffal (may Allah be pleased with him) reported that the Prophet (peace and blessings of Allah be upon him) said: "If a dog licks from a vessel, wash it seven times, and on the eighth, wipe it with earth."
Authentic. - Narrated by Muslim
Explanation
Since dogs are considered disgusting animals and carry a great deal of dirt and disease, Allah the Almighty has commanded that utensils from which dogs have eaten or drunk be washed seven times, the first time with soil, so that the utensils can be rinsed with water afterwards, thus achieving complete cleanliness and removing any impurity or potential harm.