Musannaf Ibn Abi Shayba 7:434

Waki’ narrated to us, from Jarir ibn Hazim, from Ya’la ibn Hakim, from Sa’id ibn Jubayr, who said: I said to him: A man broke his fast on a day of Ramadan intentionally, what is his expiation? He said: I do not know what his expiation is. It is a sin he committed; he should seek forgiveness from Allah and fast a day in its place.