Musnad Ahmad 25:726

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Dinar, from Tawus, who narrates from Ibn 'Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the selling of food until one has taken possession of it, or it has been taken possession of." And Ibn 'Abbas said: "I consider all sales to be in the same category as it."