Musannaf Ibn Abi Shayba 13:2786

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, he said: Yahya bin Sa'id informed us, from 'Awn bin 'Ubayd Allah bin Abi Rafi', from 'Ubayd Allah bin 'Abd Allah, he said: 'Umar bin al-Khattab said: "When the boundaries are established and people know their boundaries, then there is no pre-emption (shuf'ah) between them."