Musannaf Ibn Abi Shayba 35:343

Al-Thaqafi narrated to us, from Bard, from Makhul, who said: "There is no Zakat on horses, except for Sadaqat al-Fitr." And he mentioned that Abu Hanifa said: "If there are horses among which there are males and females, and their offspring are sought, then there is Zakat upon them."