Musannaf Ibn Abi Shayba 5:345

'Abd al-Rahim ibn Sulayman narrated to us, from Yahya ibn Sulayman, from Yahya ibn Sa'id, from 'Abd Allah ibn Dinar, who said: I asked Sa'id ibn al-Musayyib about the Zakat (charity) on pack horses. He said to me: "Is there Zakat on horses? Is there Zakat on horses?"