Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2611
Abu Haraira (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: The most excellent fast after Ramadan is God's month. al-Muharram, and the most excellent prayer after what is prescribed is prayer during the night.