Ali bin Abd al-Aziz said that Abu Ubayd (may Allah have mercy on him) said, regarding the hadith of the Prophet (peace and blessings be upon him): "There is no zakah on al-jabhah, al-kus'ah or al-nukhkhah."
Abu Ubayd said: Abu Ubaydah said: Al-jabhah is horses, al-nukhkhah is slaves and al-kus'ah is donkeys. Al-Kisa'i and others said the same about al-jabhah and al-kus'ah. Al-Kisa'i said: It is al-nukhkhah, with a damma on the nun; and he and others in his gathering explained it as working cattle.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7411