Yahya related to me from Malik that Abdullah ibn Dinar said, "Iasked Said ibn al-Musayyab about zakat on work-horses, and he said,'Is there any zakat on horses ?' " (ES)
USC-MSA web (English) reference: Book 17, Hadith 41 (ES)
Arabic reference (ES): Book 17, Hadith 617 (ES)