Musnad Ahmad 27:1694

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