Musnad Ahmad 35:5

Yahya ibn Sa'id narrated to us, from Shu'bah, Abu Bishr narrated to us, from Yusuf ibn Mahak, from Hakim ibn Hizam, who said: I said: "O Messenger of Allah, goods are requested from me, and I do not have them; should I sell them to him?" He said: "Do not sell what you do not have."