Musnad Ahmad 125:1

Hashim narrated to us, he said: Abu Bishr informed us, from Yusuf ibn Mahak, from Hakim ibn Hizam, who said: I said: "O Messenger of Allah, a man comes to me asking me to sell him something that I do not have in my possession, then I sell it to him from the market." He said: "Do not sell what you do not have."