Sunan Ibn Majah, Chapter: The Chapters Regarding Zakat

the Messenger of Allah (saw) said: "For forty sheep, one sheep up to one hundred and twenty. If there is one more. Then two sheep, up to two hundred. If there is one more, then three sheep, up to three hundred. If there are more than that, then for every hundred one sheep. Do not separate combined flock and do not combine separate flocks for fear of Sadaqah. Each partner (who has a share in the flock) should pay in proportion to his shares. And the Zakat collector should not accept any decrepit or defective animal, nor any male goat, unless he wishes to." Sahih