Fasting in Shawwal & Muharram

[Sahih Muslim : Book 6 Kitab Al-Sawm, Number 2614]
Abu Ayyub al-Ansari (Radi Allah Anhu) reported Allah's Messenger (sal-allahu- alleihi-wasallam ) as saying: He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.

[Sahih Muslim : Book 6 Kitab Al-Sawm, Number 2611]
Abu Haraira (Radi Allah Anhu) reported Allah's Messenger (sal-allahu- alleihi-wasallam ) 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 Kitab Al-Sawm, Number 2612]
Abu Huraira (Radi Allah Anhu) reported that He (sal-allahu- alleihi-wasallam ) was asked as to which prayer was most excellent after the prescribed prayer, and which fast was most excellent after the month of Ramadan. He said: Prayer offered in the middle of the night and the most excellent fast after (fasting) in the month of Ramadan is the fast in God's month al-Muharram.