Musnad Ahmad 792:26

Isma'il 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 pasturing camel, in every forty, 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 from him and half of his wealth." And he said once: "His camels are a determination from the determinations of our Lord; it is not lawful for the family of Muhammad to have anything from it."