Sulayman bin Yasar (may Allah have mercy on him) said that the people of Sham said to Abu Ubaydah bin al-Jarrah (may Allah be pleased with him): Collect zakah from our horses and slaves as well. He refused. Then he sent a letter to Umar bin al-Khattab (may Allah be pleased with him), and he too refused. The people spoke to him about it again, so he wrote to Umar bin al-Khattab (may Allah be pleased with him) once more, and Umar (may Allah be pleased with him) wrote to him: If they like to give it, take it from them and return it to them, and benefit their slaves. Malik (may Allah have mercy on him) said: That is, return it to the poor.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7413