It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked: "Which prayer is best after the obligatory prayer, and which fasting is best after the month of Ramadan?" He said: "The best prayer after the obligatory prayer is prayer in the depth of the night, and the best fasting after the month of Ramadan is fasting in Muharram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8423