Answered by Shaykh Yusuf Weltch
If someone needs to fast because they want to make up the missing days from Ramadan, for example, or because they made a false oath and they want to fast for three days to repent. Still, they don’t want their parents to know the reason (because this way, they would expose their sins).
Is it ok to pretend that you are just fasting as a Sunna in front of them and fast on Monday and Thursday? Still, the truth is that you are fasting for another reason, like the examples I mentioned above. Is their fasting still accepted? (Please keep in mind that this person isn’t fasting or pretending that they are doing a Sunnah to get anyone’s attention, they are just doing that because they need to obey Allah)
In the Name of Allah, the Most Merciful and Compassionate
Yes. It is permissible to fast an obligatory fast but conceal it from others by making it seem that one is fasting a Sunna fast instead.
Concealing One’s Sins
Concealing one’s sins is a matter encouraged in the Sacred Law. There are many examples where one can forgo a Sunna action if they fear exposing their sin.
The Messenger of Allah (Allah bless him and give him peace) said, “All of my nations are pardoned except those who publicize their sins. Indeed publicizing sins is for a person to do a (bad) deed by night, they then arise in the morning, and Allah has veiled them; thereafter, they announce, “O So and So, last night I did the such and such (bad) deed.” He had slept in Allah’s veil and woke up and removed the veil Allah put over him.” [Ahmad]
An example is that the scholars allowed a person to leave, raising their hands for the additional Takbir of Witr prayer (an emphasized Sunna action) if there was fear that people would come to know that they missed Witr and are now making it up.
Concealing Without Lying
What you want to avoid is lying. If avoidable, one must abstain from lying to conceal their sins. Fortunate for you, you can combine intentions in your obligatory make-up fast. Intend to make up the fast and to do so on a Sunna day, such as Monday or Thursday. [Ibn Nujaym; al-Ashbah wa al-Nazair]
In this manner, you are fasting an obligatory and a Sunna fast at once. So if someone asks you what you are fasting, it is not a lie to say, ‘I am fasting a Sunna fast.’
Hope this helps
Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani
وَقَال صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: كُل أُمَّتِي مُعَافًى إِلاَّ الْمُجَاهِرِينَ، وَإِنَّ مِنَ الْمُجَاهَرَةِ أَنْ يَعْمَل الرَّجُل بِاللَّيْل عَمَلاً ثُمَّ يُصْبِحَ وَقَدْ سَتَرَهُ اللَّهُ فَيَقُول: يَا فُلاَنُ عَمِلْتُ الْبَارِحَةَ كَذَا وَكَذَا، وَقَدْ بَاتَ يَسْتُرُهُ رَبُّهُ وَيُصْبِحُ يَكْشِفُ سِتْرَ اللَّهِ عَلَيْهِ.