Musnad Ahmad 27:521

Husayn ibn Ali narrated to us, from Za'idah, from Ibrahim ibn Muhajir, from Abu al-Sha'tha', who said: We came to Ibn Umar on the middle day of the days of Tashriq. He said: Food was brought, and the people drew near, but a son of his held back. He said: So he said to him, "Draw near and eat." He said: He replied, "I am fasting." He said: So he said, "Do you not know that the Messenger of Allah, may Allah bless him and grant him peace, said: 'They are days of eating and remembrance'?"