Musnad Ahmad 35:1

Hashim ibn Bashir narrated to us, 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, then I sell it to him from the market." He said: "Do not sell what you do not have."