Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2382
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: Do not observe fast for a day, or two days ahead of Ramadan except a person who is in the habit of observing a particular fast; he may fast on that day.