Musnad Ahmad 1173:11

'Affan narrated to us, he said: 'Abd al-Wahid ibn Ziyad narrated to us, he said: Khalid ibn Dhakwan narrated to us, he said: Rubayyi' bint Mu'awwidh narrated to me, she said: The Messenger of Allah (may Allah bless him and grant him peace) sent [a messenger] to the villages of the Ansar, and he said: "Whoever among you is fasting, let him complete his fast, and whoever has eaten, let him fast the remainder of his day."