Musnad Ahmad 30:2191

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from 'Abdullah ibn Dinar, he said: I heard Sulayman ibn Yasar narrating from 'Arak ibn Malik, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no charity (zakat) due on the slave of a Muslim, nor on his horse."