Musnad Ahmad 435:64

Sufyan narrated to us, from al-Zuhri, from Humayd ibn Abd al-Rahman ibn Mu'awiyah, who heard Mu'awiyah saying in Medina, upon the pulpit of the Messenger of Allah (peace and blessings of Allah be upon him): "Where are your scholars, O people of Medina? I heard the Messenger of Allah (peace and blessings of Allah be upon him) on this day, the day of Ashura, saying: 'Whoever among you wishes to fast it, let him fast it.'" And I heard the Messenger of Allah (peace and blessings of Allah be upon him) forbidding the likes of this. He then took out a lock of hair from his sleeve and said: "The Children of Israel only perished when their women adopted this."