Musnad Ahmad 30:1414

'Affan narrated to us, Abu 'Awanah narrated to us, from Abu Bishr, from Humayd ibn 'Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best fasting after Ramadan is the month of Allah, al-Muharram. And the best prayer after the obligatory [prayer]—or the obligatory—is the night prayer."