Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The best fasting after Ramadan is that of the sacred month of Allah, Muharram, and the best prayer after the obligatory prayers is the night prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8421