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