Al-Ma'idah' - 5:89

لاَ يُؤَاخِذُكُمُ اللّهُ بِاللَّغْوِ فِي أَيْمَانِكُمْ وَلَـكِن يُؤَاخِذُكُم بِمَا عَقَّدتُّمُ الأَيْمَانَ فَكَفَّارَتُهُ إِطْعَامُ عَشَرَةِ مَسَاكِينَ مِنْ أَوْسَطِ مَا تُطْعِمُونَ أَهْلِيكُمْ أَوْ كِسْوَتُهُمْ أَوْ تَحْرِيرُ رَقَبَةٍ فَمَن لَّمْ يَجِدْ فَصِيَامُ ثَلاَثَةِ أَيَّامٍ ذَلِكَ كَفَّارَةُ أَيْمَانِكُمْ إِذَا حَلَفْتُمْ وَاحْفَظُواْ أَيْمَانَكُمْ كَذَلِكَ يُبَيِّنُ اللّهُ لَكُمْ آيَاتِهِ لَعَلَّكُمْ تَشْكُرُونَ
[5:89]
Allah does not call you to account for what is vain in your oaths, but He calls you to account for the making of deliberate oaths; so its expiation is the feeding of ten poor men out of the middling (food) you feed your families with, or their clothing, or the freeing of a neck; but whosoever cannot find (means) then fasting for three days; this is the expiation of your oaths when you swear; and guard your oaths. Thus does Allah make clear to you His communications, that you may be Fateful.
Allah will not call you to account for what is futile in your oaths, but He will call you to account for your deliberate oaths: for expiation, feed ten indigent persons, on a scale of the average for the food of your families; or clothe them; or give a slave his freedom. If that is beyond your means, fast for three days. That is the expiation for the oaths ye have sworn. But keep to your oaths. Thus doth Allah make clear to you His signs, that ye may be grateful.
Allah will not take you to task for that which is unintentional in your oaths, but He will take you to task for the oaths which ye swear in earnest. The expiation thereof is the feeding of ten of the needy with the average of that wherewith ye feed your own folk, or the clothing of them, or the liberation of a slave, and for him who findeth not (the wherewithal to do so) then a three days' fast. This is the expiation of your oaths when ye have sworn; and keep your oaths. Thus Allah expoundeth unto you His revelations in order that ye may give thanks.