Musannaf Ibn Abi Shayba 5:131

Muhammad ibn Bakr narrated to us, from Ibn Jurayj, from Salih ibn Dinar, that Umar ibn Abd al-Aziz wrote to Uthman ibn Muhammad ibn Abi Suwayd: "Take from every thirty cows a tabi' (a one-year-old calf), and from every forty cows a cow." And he did not add anything to that.