Musnad Ahmad 30:1239

Al-Husayn ibn Ali al-Ju'fi narrated to us, from Za'idah, from 'Abd al-Malik ibn 'Umayr, from Muhammad ibn al-Muntashir, from Humayd ibn 'Abd al-Rahman al-Himyari, from Abu Hurayrah, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, which prayer is best after the obligatory one?" He said: "Prayer in the middle of the night." He said: "Then which fasting is best after Ramadan?" He said: "The month of Allah, which you call al-Muharram."