Musnad Ahmad 30:669

'Abd al-Razzaq narrated to us, Ma'mar and 'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to encourage the standing of Ramadan, without commanding them with a firm command. He would say: "Whoever stands [in prayer during] Ramadan out of faith and seeking reward, his past sins will be forgiven."