Sayyiduna Bilal (may Allah be pleased with him) said: I had some dates belonging to the Messenger of Allah (peace and blessings be upon him), and I found dates better than them at one sa' for two sa's. So I bought them and brought them to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said: "What is this?" I said: I took one sa' for two sa's. He (peace and blessings be upon him) said: "Return our dates to us."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1093