Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him): What is ihtikar (hoarding)? He (peace and blessings be upon him) said: "When he hears of prices falling it displeases him, and when he hears of prices rising he is glad. What an evil servant the hoarder is! If Allah makes prices cheap he grieves, and if He makes them dear he rejoices."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17322