Musnad Ahmad 32:66

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) arrived in Medina, and they had two days in which they used to play during the Jahiliyyah (pre-Islamic period). He said: "Indeed, Allah has replaced them for you with two that are better than them: the day of Al-Fitr and the day of Al-Nahr."