Musnad Ahmad 830:112

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Khalid al-Hadhdha' narrating from 'Abd al-Rahman ibn Abi Bakrah, from his father, from the Prophet, peace and blessings of Allah be upon him, who said: "Two months do not decrease, in each of them there is an 'Eid: Ramadan and Dhu al-Hijjah."