Musnad Ahmad 30:3057

Waki' narrated to us, he said: Sufyan and Shu'bah narrated to us, from 'Abdullah ibn Dinar, from Sulayman ibn Yasar, from 'Arak ibn Malik, from Abu Hurayrah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Zakat (charity) upon the Muslim regarding his horse, nor his slave."