Abu Bakr bin Muhammad bin Amr bin Hazm (may Allah have mercy on him) narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) sent a letter to the people of Yemen in which it was written: "In every thirty cattle there is a male or female calf, and in every forty cattle one cow."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7296