Musnad Ahmad 830:27

Isma'il narrated to us, from Khalid al-Hadhdha', from 'Abd al-Rahman ibn Abi Bakrah, from his father, who said: I think he said, from the Prophet, peace and blessings of Allah be upon him, who said: "Two months do not decrease, the months of 'Eid: Ramadan and Dhu al-Hijjah."