Humayd (may Allah have mercy on him) narrates that Sayyiduna Mu'awiyah (may Allah be pleased with him) said on the pulpit of Madinah: O people of Madinah! Where are your scholars? The Messenger of Allah (peace and blessings be upon him) used to forbid the like of this. And he took out a lock of hair from his sleeve and said: The Children of Israel were destroyed only when their women adopted the like of this. O people of Madinah! Where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) on this day (the day of Ashura) saying: "I am fasting; whoever of you wishes to fast, let him fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8417