Musannaf Ibn Abi Shayba 13:1348

Abu Bakr narrated to us, he said: Ibn al-Mubarak narrated to us, from Ma’mar, from al-Zuhri, he said: "At-Tawliyah (selling at cost price) is a sale. Do not sell (the item) by Tawliyah until you have taken possession of it."