Musannaf Ibn Abi Shayba 13:1345

Abu Bakr narrated to us, he said: Humayd ibn Abd al-Rahman narrated to us, from Hasan, from Mujalid, from al-Sha’bi, who said: For the absent person, there is a right of pre-emption (shuf’ah). It is to be written to him; if he takes it and sends the price, then [he has the right], otherwise he has no right of pre-emption.