Dawud from al-Sha'bi (may Allah have mercy on him), and Ibn Abi Ayyash from Anas (may Allah be pleased with him), narrate in marfu' form that he (peace and blessings be upon him) said: "In forty cattle there is one musinnah, and in thirty cattle one male or female yearling calf."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7297