Musnad Ahmad 27:997

'Affan narrated to us, Abu 'Awanah narrated to us, Yazid ibn Abi Ziyad narrated to us, from Mujahid, from Ibn 'Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "There are no days greater in the sight of Allah, nor more beloved to Him for deeds to be done in them, than these ten days. So increase in them the declaration of the oneness of Allah (tahlil), the declaration of the greatness of Allah (takbir), and the declaration of the praise of Allah (tahmid)."