Suwayd bin Ghafalah (may Allah have mercy on him) said: The collector of the Messenger of Allah (peace and blessings be upon him) came to us, so I came and sat with him. I heard him saying: "Included in my instructions is that I do not take a milk-giving animal, nor combine those grazing separately, nor separate those grazing together." Then a man brought him a she-camel with a large hump and said: "Take this," but the collector refused to take it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7305