Musnad Ahmad 30:3122

Surayj ibn al-Nu‘man narrated to us, Fulayh narrated to us, from Hilal, from ‘Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you cleans himself with stones (istijmar), let him use an odd number. And when a dog licks the vessel of one of you, let him wash it seven times. And do not withhold surplus water so as to withhold pasture thereby. And it is the right of the camels that they be milked at the water on the day they come to drink."