It is narrated from Sayyiduna Mu'awiyah bin Haydah (may Allah be pleased with him); he said: I heard the Prophet (peace and blessings be upon him) saying: "In every herd of grazing camels, for every forty a bint labun (two-year-old she-camel) is due as zakah. Whoever gives it seeking the reward will have its reward, and whoever withholds it, we will take it along with half of his camels. This is one of the firm commands of our Lord. Nothing of it is lawful for the family of Muhammad."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17035