Musannaf Ibn Abi Shayba 13:1443

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from his father, who said: ‘Amir was asked about the *salam* (forward sale) in wheat and barley: "Whichever of the two is easier for him, he gives it." He said: "It is not permissible."