Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that he (peace and blessings be upon him) forbade riding an animal that eats filth (jallalah).
(b) Amr bin Abi Qays narrates from Ayyub (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) forbade it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10330