Musnad Ahmad 792:23

Yahya ibn Sa'id narrated to us, Bahz narrated to us, my father narrated to me, from my grandfather, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "In every pasturing camel, for every forty, there is a *bint labun* (a two-year-old female camel). Camels shall not be separated from their count. Whoever gives it seeking reward, then for him is its reward. And whoever withholds it, then we shall take it and half of his camels. It is a determination from the determinations of our Lord; nothing of it is lawful for the family of Muhammad."