Musannaf Ibn Abi Shayba 13:2349

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Alqamah ibn Marthad, from Razin ibn Sulayman, who said: There is no harm in *salam* (forward sale) regarding food, with a known measure, for a known term.