Musannaf Ibn Abi Shayba 13:3234

Abu Bakr narrated to us, he said: Yahya ibn Sa'id al-Qattan narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar, that 'Umar reserved al-Rabadhah for the camels of the charity (zakat).