Maliks Muwatta Book 21, Hadith Number 21.
Yahya related to me that Malik said that he had heard that Umar ibn Abd al-Aziz used to say, “The man on horse-back has two shares, and the man on foot has one.”
Malik added, “I continue to hear the same.”
Malik, when asked whether a man who was present with several horses took a share for all of them, said, “I have never heard that. I think that there is only a share for the horse on which he fought.”
Malik said, “I think that foreign horses and half-breeds are considered as horses because Allah, the Blessed, the Exalted, said in His Book, ‘All horses, and mules, and asses, for you to ride, and as an adornment.’ (Sura16 ayat 8). He said, the Mighty, the Majestic, ‘Make ready for them whatever force and strings of horses you can, to terrify thereby the enemy of Allah and your enemy.’ (Sura 8 ayat 60). I think that foreign breeds and half-breeds are considered as horses if the governor accepts them.”
Said ibn al-Musayyab was asked about working horses, and whether there was zakat on them. He said, “Is there any zakat on horses.?”