Musnad Ahmad 3881

It was narrated from Abu "Ubaidah that his father said: The Messenger of Allah (ﷺ) wrote concerning zakah on cattle. “When (the number of cattle) reaches thirty, a one year old, male or female, is due (as zakah), up to forty. When (the number) reaches forty, a two year old, male or female, is due (as zakah). If the number of cattle is great, then for every forty, a two year old cow is due (as zakah)."