Musnad Abu Ya'la al-Mawsili 159:147

Ishaq ibn Abi Isra'il narrated to us, he said: Jarir ibn 'Abd al-Hamid narrated to me, from 'Abd al-Malik ibn 'Umayr, from Muhammad ibn al-Muntashir, from Humayd ibn 'Abd al-Rahman, from Abu Hurayrah, who attributed it to the Prophet (ﷺ), he said: It was asked: "Which prayer is best after the obligatory one, and which fasting is best after Ramadan?" He said: "The best prayer after the obligatory one is prayer in the middle of the night, and the best fasting after Ramadan is the month of Allah, al-Muharram."