Musnad Ahmad 792:1

Isma'il ibn 'Ulayyah narrated to us, from Bahz ibn Hakim, from his father, from his grandfather, who said: I heard the Prophet of Allah, peace and blessings of Allah be upon him, saying: "In every grazing camel, in every forty, there is a daughter of a milch camel (bint labun). 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 from him 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."