Musnad Abu Ya'la al-Mawsili 7:31

Hadba narrated to us, al-Qasim ibn al-Fadl narrated to us, al-Nadr ibn Shayban narrated to us, he said: I was at Arafat and I met Abu Salamah ibn Abd al-Rahman, so I said: "Narrate to me something you heard from your father, with no one between your father and the Messenger of Allah, may Allah bless him and grant him peace." He said: My father narrated to me from the Messenger of Allah, may Allah bless him and grant him peace, that he said: "Indeed, Allah has obligated the fasting of Ramadan, and I have established its standing (in prayer)."