Sahih Muslim Book 6, Hadith Number 2606.

Chapter : Excellence of observing fast for three days during every month, and fasting on the day of ‘Arafa, and ‘Ashura and Monday and Thursday.

Abu Qatada Ansari (Allah be pleased with him) reported that Allah’s Messenger (may peace be upon him) was asked about fasting on Monday, whereupon he said: It is (the day) when I was born and revelation was sent down to me.