Musannaf Ibn Abi Shayba 5:94

Ya'la ibn 'Ubayd narrated to us, from Yahya ibn Sa'id, who said: It was in the document that 'Umar ibn 'Abd al-'Aziz wrote to them when he sent them to collect the Zakat (Sadaqat) on camels: When they reach twenty-five, then in them is a bint makhad (a one-year-old female camel). If they increase, then an ibn labun (a two-year-old male camel).