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 in the month of Allah that you call al-Muharram, and the best prayer after the obligatory prayer is the prayer in the middle of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8422