Musannaf Ibn Abi Shayba 3:5328

Waki' narrated to us, from Ali, from Nadr ibn Shayban, who said: I asked Abu Salamah ibn Abd al-Rahman. He mentioned from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah has made its fasting obligatory upon you, and I have established its standing (in prayer) for you. So whoever fasts it out of faith and seeking reward, his past sins will be forgiven."