Musnad Ahmad 30:3712

Uthman ibn Umar narrated to us, Malik narrated to us, from al-Zuhri, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to encourage the standing of Ramadan without commanding it as an obligation. He would say: "Whoever stands [in prayer during] Ramadan out of faith and seeking reward, his past sins will be forgiven."