Fasting Ramadaan is one of the pillars of Islam, and it is not permissible for one who is required to fast not to do so, unless he has an excuse. Whoever does not fast due to a legitimate shar’i excuse, such as sickness, travelling or menstruation, and is able to fast, has to make up the missed fasts, because Allaah says (interpretation of the meaning):
“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days”
The one who deliberately does not fast with no excuse is not like one who has an excuse in this regard.
If a person delays an act of worship from the time when it is due, such as prayer or fasting, with no excuse, it is not valid and will not be accepted if he does it after the time specified for it in sharee’ah is over.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked:
What is the ruling on a Muslim who did not fast Ramadaan for many years although he did all the other duties required of him, and he has no reason why he should not fast. Does he have to make up those fasts if he repents?
The correct view is that he does not have to make up those fasts if he repents, because with regard to every act of worship that is prescribed at a specific time, if a person delays it beyond that time deliberately with no excuse, Allaah will not accept it from him. So there is no point in making them up. But he has to repent to Allaah and do a lot of righteous deeds. Whoever repents, Allaah accepts his repentance.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 19, question no. 41
This is the ruling on one who does not fast and has no excuse for that, i.e., he did not have any intention of fasting and he did not start fasting at all.
With regard to one who starts a fast, then during the day he breaks the fast, he has to make up that day.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about the ruling on breaking the fast during the day in Ramadaan with no excuse.
Breaking the fast during the day in Ramadaan with no excuse is a major sin, which makes a person a faasiq (rebellious evildoer). He has to repent to Allaah and make up the day when he broke the fast. This means that if he fasted then during the day he broke the fast with no excuse, then he is a sinner, and he has to make up that day when he broke the fast, because when he started the fast it became binding upon him to complete it, as in the case of a vow. But if he deliberately did not start to fast at all with no excuse, then he does not have to make it up, because that will be of no benefit to him, for it will never be accepted from him. The basic principle with regard to every act of worship that is connected to a specific time is that if it is delayed beyond that time with no excuse, it will not be accepted, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does an action that is not in accordance with this matter of ours will have it rejected.” And this is a transgression of the sacred limits of Allaah, and transgressing the sacred limits of Allaah is zulm (wrongdoing), and the wrongdoer’s deeds are not accepted. Allaah says (interpretation of the meaning):
“And whoever transgresses the limits ordained by Allaah, then such are the Zaalimoon (wrongdoers)”
If he did this act of worship before the time for it began, it would not be accepted from him, and by the same token if he does it after the time for it is over, it will not be accepted from him, unless he had an excuse.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 19, question no. 45.
And Allaah knows best.