Musannaf Ibn Abi Shayba 13:894

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Abu 'Awanah, from Dawud ibn 'Abdullah, from Abu al-Mukhariq, from Abu Hurayrah, who said: The Muslims made peace [regarding Salam transactions], so whoever enters into a Salam contract for wheat, let him not take barley [instead]. And whoever enters into a Salam contract for wheat, [it must be] by a known measure, for a specified term.