Silsilat al-Ahadith al-Sahihah

Sayyiduna Anas (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) forbade fasting on six days in the year: the three days of Tashreeq, the day of Eid al-Fitr, the day of Eid al-Adha, and specifically fasting on Friday.