Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "There are no days greater in the sight of Allah than these ten days, nor are deeds in any days more beloved to Him than in them, so recite abundantly in them tasbih (glorifying Allah), takbir (declaring His greatness) and tahlil (declaring His oneness)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11070