Ubaydullah bin Umar (may Allah have mercy on him) said: I asked Nafi' about cattle, and he said: It has reached me from Mu'adh bin Jabal (may Allah be pleased with him) that he said: In every thirty cattle there is a male or female yearling calf, and in every forty cattle there is one cow.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7289