Musannaf Ibn Abi Shayba 13:2347

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Shu’bah narrated to us, from Abu ‘Umar al-Bahrani, Yahya bin ‘Ubayd, he said: I heard Ibn ‘Abbas saying: There is no harm in *as-salam* (forward sale) regarding food, with a known measure, to a known term.