Suwayd bin Ghafalah (may Allah have mercy on him) said: The zakah collector of the beloved Prophet (peace and blessings be upon him) came to us. I took him by the hand and read in his written document: "Those (animals) that are separate shall not be combined, nor shall those that are combined be separated, out of fear of zakah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7332