Musnad Ahmad 35:2

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Abu Bishr, from Yusuf ibn Mahak, who narrated from Hakim ibn Hizam, who said: I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, that I would not fall except while standing. He said: I said, "O Messenger of Allah, a man asks me to sell something, and I do not have it; should I sell it to him?" He said, "Do not sell what you do not have."