Musnad Ahmad 30:763

Isma'il ibn 'Umar narrated to us, Ibn Abi Dhi'b narrated to us, from Ibn Shihab, from Abu Salamah ibn 'Abd al-Rahman, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, encouraging the people to perform the night prayer (qiyam) of Ramadan, and he would say: "Whoever performs the night prayer of it out of faith and seeking reward, his past sins will be forgiven." And the Messenger of Allah, peace and blessings of Allah be upon him, did not gather the people for the night prayer.