Musnad Ahmad 27:1441

Ishaq ibn Isa narrated to us, Ibn Lahi'ah informed us, from Abu al-Aswad, from al-Qasim ibn Muhammad, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever buys food by measure or by weight, let him not sell it until he takes possession of it."