Bahz bin Hakim narrates from his father, and he from his grandfather (may Allah be pleased with him), that he said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "In grazing camels, on every forty there is a bint labun (a she-camel in its third year), and camels are not to be separated from their count. Whoever gives it seeking reward will have its reward, and whoever withholds it, we will take it from him along with half of his camels, as one of the firm decrees of our Lord. Nothing of it is lawful for the family of Muhammad (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7390