Musnad Ahmad 1068:0

Narrated to us by Yahya ibn Sa’id, from Hisham, who said: My father narrated to me, from Nawfal ibn Mu’awiyah, who said: I said: "O Messenger of Allah, I have one hundred sheep, and I graze them." He said: "Pay their Zakat." I said: "I have more than that." He said: "For every forty, there is one sheep, until you reach one hundred and twenty. If they exceed one hundred and twenty by one, then there are two sheep in them, until you reach two hundred. If they exceed two hundred by one, then there are three sheep in them, until you reach three hundred. If they exceed three hundred, then for every hundred, there is one sheep. Do not mix the separate, and do not separate the combined, out of fear of Zakat. What is shared between two partners, they shall return to each other with equity."