Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2606
Abu Qatada Ansari (Allah be pleased with him) reported that Allah's Massenger (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.