Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) says: One day I became aroused and kissed my wife while I was fasting, and I came to the Prophet (peace and blessings be upon him). I said: "Today I have done a very grave thing: I kissed while fasting." The Messenger of Allah (peace and blessings be upon him) said: "What do you think if you rinsed your mouth with water while fasting?" He says: I said: "There is no harm in that." The Messenger of Allah (peace and blessings be upon him) said: "Then what is troubling you?"
Imam al-Shafi'i (may Allah have mercy on him) says: If he swallowed it after dawn, he makes up a day in its place.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8018