Musnad Abu Ya'la al-Mawsili 155:129

Abu Bakr ibn Abi Shaybah narrated to us, 'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah: That the Prophet (peace and blessings of Allah be upon him) used to encourage the standing [in prayer] of Ramadan without making it an obligation.