Abd al-Malik relates that Sayyiduna Jundub bin Sufyan (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) used to say: "The best prayer after the obligatory prayer is the prayer in the middle of the night, and the best fasting after Ramadan is (fasting in) the month of Allah which you call Muharram."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1682