Ibrahim (al-Nakha'i) and Sa'id bin Jubayr (may Allah have mercy on them) said about a man who deliberately broke one day's fast in Ramadan: We do not know what his expiation is; he should fast one day in its place, and repent and seek forgiveness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8068