Musannaf Ibn Abi Shayba 5:139

Mu'awiyah ibn Hisham narrated to us, from Sufyan, from Ibn Abi Layla, from 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu Sa'id al-Khudri, who said: "There is nothing [due as Zakat] on fewer than thirty cows."