The most excellent fast after Ramadan is God's month

English Translation:

✦ Abu Hurairah 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. 

Sahih Muslim, Book 6, Hadith 2611 

✦ It was narrated that Abu Hurairah may Allah be pleased with him said: A man came to the Prophe Peace be upon him and said: Which fasting is best after the month of Ramadan? He Peace be upon him said: The month of Allah which is called Muharram. 

Sunan Ibn Majah, Vol 1 , # 1742 (Sahih)