Musannaf Ibn Abi Shayba 13:545

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-Hakam ibn Abi al-Fadl, he said: I said to al-Hasan: "A man comes to me and bargains with me for silk that I do not have. So I go and get the item (that was bargained for), then I sell it to him." He said: "This is 'al-muwasafah' (specifying/describing goods not in possession)," and he disliked it.