I heard the Prophet ﷺ say, "For every herd of grazing camels, in every forty there is due a two-year-old female camel. Camels are not to be separated from their count. Whoever gives it seeking reward, he will have its reward, and whoever refuses, we will take it from him along with half of his camels, as a binding right from the rights of our Lord. Nothing of it is lawful for the family of Muhammad ﷺ."
أخبرنا عمرو بن علي قال حدثنا يحيى قال حدثنا
سمعت النبي ﷺ يقول في كل إبل سائمة في كل أربعين ابنة لبون لا يفرق إبل عن حسابها من أعطاها مؤتجرا فله أجرها ومن أبى فإنا آخذوها وشطر إبله عزمة من عزمات ربنا لا يحل لآل محمد ﷺ منها شيء
Its isnad is hasan. Abu Dawud narrated it in the Book of Zakat, chapter on the zakat of grazing livestock, hadith 1575, from the account of the hadith of Bahz, and it is also in Kubra, hadith 2224. Ibn Khuzayma graded it sahih, hadith 2266, as did Hakim, 1/398, and Dhahabi.