Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: When the Messenger of Allah (peace and blessings be upon him) sent Sayyiduna Mu'adh (may Allah be pleased with him) to Yemen, he ordered him to take from every thirty cattle a male or female yearling calf that is a jadha'ah, and from every forty cattle a musinnah. They said: What is the ruling about the awqas? He said: He did not command me anything about them, and I will ask the Messenger of Allah (peace and blessings be upon him) when I go to him (peace and blessings be upon him). So when he came to the Messenger of Allah (peace and blessings be upon him), he asked him about the awqas, and he (peace and blessings be upon him) said: "There is no zakah in them." Al-Mas'udi said: The awqas are what is below thirty and what is between forty and sixty. When they reach sixty, there are two female yearling calves in them; when they reach seventy, there is one musinnah and one yearling calf; when they reach eighty, there are two musinnahs; and when they reach ninety, there are three yearling calves. Al-Mas'udi said: Awqas is (spelled) with sin, so do not take it as being with sad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7293