Bukhari

And it is narrated from Abu Huraira (may Allah be pleased with him) in a marfu‘ (attributed to the Prophet) manner that if someone does not fast for even one day in Ramadan without a valid excuse or illness, then fasting for the whole lifetime would not compensate for it. And this is also the statement of Ibn Mas‘ud (may Allah be pleased with him). And Sa‘id ibn al-Musayyib, al-Sha‘bi, Ibn Jubayr, Ibrahim, Qatadah, and Hammad (may Allah have mercy on them) also said that one should fast one day in compensation for it.

Narrated `Aisha: A man came to the Prophet and said that he had been burnt (ruined). The Prophet asked him what was the matter. He replied, "I had sexual intercourse with my wife in Ramadan (while I was fasting)." Then a basket full of dates was brought to the Prophet and he asked, "Where is the burnt (ruined) man?" He replied, "I am present." The Prophet told him to give that basket in charity (as expiation).

And the Prophet (peace and blessings of Allah be upon him) did not differentiate between the fasting and the non-fasting person. And Imam Hasan al-Basri said that there is no harm in putting (medicine, etc.) into the nose as long as it does not reach the throat, and a fasting person may also apply kohl (antimony). Ata’ said that if one rinses the mouth and expels all the water from the mouth, then there is no harm, and if he does not swallow his saliva, and whatever (moisture of water) remains in his mouth, and one should not chew mastic. If someone swallows the saliva of mastic, I do not say that his fast is broken, but it is prohibited. And if someone puts water into his nose and the water (involuntarily) reaches the throat, then his fast is not broken because this matter was beyond his control.