Sayyiduna Ma'mar bin Abi Ma'mar (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "None hoards except a sinner." Ata said: I said to Sa'id (may Allah be pleased with him): Do you also practise ihtikar (hoarding)? He said: Ma'mar also used to practise ihtikar. In one narration it says that Sa'id bin al-Musayyib (may Allah be pleased with him) used to practise ihtikar in olive oil.
The Shaykh said: The details of ihtikar are explained in the books of fiqh, and my opinion about these two men is that they did not practise the forbidden kind of ihtikar. And it is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) forbade the ihtikar of wheat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11148