Sayyiduna Uthman bin Affan (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell one dinar for two dinars, nor one dirham for two dirhams." [Sahih: Muslim 1585]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10485