Abu Bakr bin Muhammad bin Amr bin Hazm narrates from his father, and he from his grandfather (may Allah be pleased with him), that in the letter which the Prophet (peace and blessings be upon him) wrote to the people of Yemen it was stated: "There is nothing (no zakah) on a Muslim's slave or his horse."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7408