Musnad Ahmad 30:907

'Abd al-Rahman and Abu Sa'id narrated to us, they said: Za'idah narrated to us, 'Abd al-Malik ibn 'Umayr narrated to us, from Muhammad ibn al-Muntashir, from Humayd ibn 'Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked: "Which prayer is best after the obligatory ones?" He said: "Prayer in the middle of the night." It was said: "Which fasting is best after Ramadan?" He said: "The month of Allah, which you call al-Muharram."