Sa'id bin Jubayr narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "What deed done on ordinary days is more virtuous and better than in the ten days of Dhu al-Hijjah? There is no deed done on ordinary days that is more virtuous than in the ten days of Dhu al-Hijjah." They said: O Messenger of Allah (peace and blessings be upon him)! Not even jihad in the path of Allah? He (peace and blessings be upon him) said: "Not even jihad in the path of Allah, except a man who went out with his wealth and his self (his life), and nothing of either of them came back."
Abu Mu'awiyah said: The Messenger of Allah (peace and blessings be upon him) said: "There are no days in which a righteous deed is more beloved to Allah than in these days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8392