Suwayd bin Ghafalah (a mukhadram, who lived in both eras but did not meet the Prophet) said: I travelled with the zakah collector of the Messenger of Allah (peace and blessings be upon him), or someone who travelled with him told me, and in the written covenant of the Messenger of Allah (peace and blessings be upon him) it was stated: "Zakah is not to be taken from a suckling animal, and separate holdings are not to be combined, nor combined holdings separated (to avoid zakah)."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6443
Suwayd bin Ghafalah said: The zakah collector of the Messenger of Allah (peace and blessings be upon him) came to us. I took hold of his hand and he took hold of mine, and I read in his written covenant: "Separate holdings are not to be combined, nor combined holdings separated, for fear of (paying more) zakah." Then a man brought him a large she-camel, but he refused to take it. Then he brought another, inferior to it, and he refused to take that too. Then he said: Which earth would carry me and which sky would shade me if I came to the Messenger of Allah (peace and blessings be upon him) having taken the best of a Muslim's wealth?
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6444