Musnad Ahmad 30:3174

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Ibn Shihab, from Humayd ibn Abd al-Rahman, from Abu Hurayrah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever stands (in prayer) during Ramadan out of faith and seeking reward, his past sins will be forgiven."