Sunan ad-Darimi

Bahz bin Hakim narrated from his grandfather, who narrated from his father, that he heard the Messenger of Allah (peace be upon him) say: "For every forty camels grazing in the wilderness, one bint labun (a two-year-old she-camel) is due as zakat, and the camels should not be separated from their place (to avoid zakat). Whoever pays zakat seeking reward will receive a reward, and whoever withholds zakat, we will take the zakat from him, and in addition, half of his wealth as a penalty for withholding it, because this is one of the obligations imposed by Allah the Exalted. Nothing from it is lawful for the family of Muhammad."