Musannaf Ibn Abi Shayba 35:457

Waki’ narrated to us, Isma’il bin Muslim al-‘Abdi narrated to us, Abu al-Mutawakkil al-Naji narrated to us, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Wheat for wheat, and barley for barley, like for like, hand to hand." And he mentioned that Abu Hanifah used to say: "There is no harm in selling absent wheat, specifically, for present wheat."