Musnad Ahmad 30:337

Narrated to us Rab'i, narrated to us Abd al-Rahman ibn Ishaq, from Abdullah ibn Dinar, from Sulayman ibn Yasar, from 'Arak ibn Malik, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "There is no charity (Zakat) upon the Muslim regarding his horse nor his slave."