Jundub bin Sufyan al-Bajali (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) used to say: "Indeed, the best prayer after the obligatory prayers is prayer at night, and the best fasting after the month of Ramadan is in Allah's month which you call Muharram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8424