Sayyiduna Abd al-Rahman bin Samurah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "There is no zakah on al-kus'ah, al-jabhah and al-nukhkhah." Abu Amr explained that al-kus'ah means donkeys, al-jabhah means horses and al-nukhkhah means slaves.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7410