Musnad Ahmad 5:700

'Abdullah narrated to us, Ahmad ibn Muhammad ibn Ayyub narrated to us, Abu Bakr ibn 'Ayyash narrated to us, from al-A'mash, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali: That the Messenger of Allah (peace and blessings of Allah be upon him) said: "I have pardoned for you [the Zakat on] horses and slaves, and there is no Sadaqah (Zakat) upon them."