Musnad Ahmad 30:1387

'Affan narrated to us, Abu 'Awanah narrated to us, from 'Abd al-Malik ibn 'Umayr, from Muhammad ibn al-Muntashir, from Humayd ibn 'Abd al-Rahman, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The best prayer after the obligatory one is prayer in the middle of the night. And the best fasting after the month of Ramadan is the month of Allah, which you call al-Muharram."