Sayyiduna Jundub bin Abdullah al-Bajali (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The most virtuous prayer after the obligatory prayer is that which is offered in the middle part of the night, and the most virtuous fasting after the fasting of Ramadan is that of the month which you call Muharram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4662