Imam al-Shafi'i (may Allah have mercy on him) said in his old (opinion): "Allah Almighty commanded that they prepare against their enemy whatever strength they can and tethered horses, and Allah Almighty did not single out the Arab (pure-bred) horse to the exclusion of the hybrid horse. The Messenger of Allah (peace and blessings be upon him) permitted (eating) the meat of horses, and this (permission) covered (both) the hybrid and the Arab. He (peace and blessings be upon him) said: 'We have exempted you from the zakah on horses and slaves.' And he said: 'There is no zakah due from a Muslim on his horse or his slave.' So he (peace and blessings be upon him) counted the faras (a single horse) as part of the khayl (the genus of horses)."
Imam al-Shafi'i (may Allah have mercy on him) said: "It has been mentioned about the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) gave the Arab horse preference over the hybrid (in the spoils), and that Sayyiduna Umar (may Allah be pleased with him) also did so."
Imam al-Shafi'i (may Allah have mercy on him) said: "No one narrated this except Makhul, as mursal, and in our view no proof is established by a mursal report like it. Likewise, the report of Sayyiduna Umar (may Allah be pleased with him) is also mursal from Kulthum bin al-Aqmar."
Imam al-Shafi'i (may Allah have mercy on him) said: "Hammad bin Khalid informed us from Mu'awiyah bin Salih, from Abu Bishr, from Makhul, that the Prophet (peace and blessings be upon him) declared the Arab horse to be Arab and the hybrid to be hybrid (that is, he distinguished between the shares of the two)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / Q12882