It is narrated from Sayyiduna Mu'awiyah bin Haydah (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "For every forty grazing camels, a two-year-old she-camel (bint labun) is due as zakah. Whoever gives it seeking the reward will have its reward, and whoever conceals it, we will take it along with half of his camels. This is one of the firm commands of your Lord. (This zakah wealth) is lawful neither for Muhammad (peace and blessings be upon him) nor for the family of Muhammad."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17033