Musnad Abu Ya'la al-Mawsili 151:50

Abu Bakr narrated to us, Abd al-Salam narrated to us, from Khusayf, from Abu Ubaydah, from Abdullah: That the Prophet (peace and blessings of Allah be upon him) said: "Regarding thirty cows, [the Zakat is] a Tabi' (a one-year-old male calf) or a Tabi'ah (a one-year-old female calf). And regarding forty, [the Zakat is] a Musinnah (a two-year-old cow)."