Sayyiduna Abu Sa'id (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "When you make a salaf (advance-payment) sale in something, do not transfer it to something else." In one narration the wording is: "Whoever makes a salaf sale in something should not transfer it to something else."
Reliance is placed on the hadith in which selling wheat before taking it into one's possession is forbidden, because Atiyyah al-Awfi is not taken as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11153