Hadith: Verily, if one of you insists on fulfilling an oath by which he may harm his family that is more sinful in the sight of Allah, the Almighty, than breaking the oath and paying the (oath-breaking) expiation that Allah imposed

عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «لأَنْ يَلَجَّ أحدُكم في يَمِينِه في أهْلِه آثَمُ له عند الله تعالى من أن يُعْطِي كَفَّارَتَهُ التي فرض الله عليه».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "c2">“Verily, if one of you insists on fulfilling an oath by which he may harm his family that is more sinful in the sight of Allah, the Almighty, than breaking the oath and paying the (oath-breaking) expiation that Allah imposed.”
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The meaning of this Hadīth is that if a Muslim took an oath concerning his family, whereby they will be harmed if he does not break the oath and breaking it would not constitute disobedience to Allah, the Almighty, yet he still insists on fulfilling the oath, fulfilling the oath would be a greater sin than breaking it and paying the legislated expiation for breaking the oath. In other words, he should pay the expiation for breaking his oath instead of insisting on fulfilling it, as long as breaking it does not involve disobedience to Allah. In fact insisting on fulfilling the oath is a greater sin, because it causes harm to his family, and Allah has made the matter flexible for him. Another report reported by Al-Bukhāri and Muslim reads: “If you take an oath to do something and then you find that something else is better, you should do what is better and expiate for breaking your oath."