Musannaf Ibn Abi Shayba 13:2798

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Yunus bin Abi Ishaq narrated to us, he said: I heard al-Sha’bi saying: Whoever sells his right of pre-emption (shuf’ah) while he is a witness and does not object, then there is no right of pre-emption for him.