Musnad Ahmad 1173:12

'Ali ibn 'Asim narrated to us, he said: Khalid ibn Dhakwan informed us, he said: I asked al-Rubayyi' bint Mu'awwidh ibn 'Afra' about the fasting of 'Ashura'. She said: The Messenger of Allah (may Allah bless him and grant him peace) said on the day of 'Ashura': "Who among you has woken up fasting?" He said: They said: Among us are those who are fasting, and among us are those who are not fasting. He said: "Then complete the remainder of your day, and send word to those around Medina, so that they may complete the remainder of their day."