Musnad Ahmad 30:3281

Hashim narrated to us, he said: Awf narrated to us, from a man who narrated to him from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The protected area (harim) of a well is forty cubits all around it, for the resting places of camels and sheep. And the wayfarer is the first to drink. And the surplus of water should not be withheld so that the pasture may be withheld by it."