Musannaf Ibn Abi Shayba 13:2578

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from Ash'ath, from al-Sha'bi, who said: Pre-emption (al-shuf'ah) is according to the heads of the men. And al-Hasan said: It is according to the proportion of the shares.