لَا يُؤَاخِذُكُمُ اللّٰهُ بِاللَّغْوِ فِيْٓ اَيْمَانِكُمْ وَلٰكِنْ يُّؤَاخِذُكُمْ بِمَا عَقَّدْتُّمُ الْاَيْمَانَۚ فَكَفَّارَتُهٗٓ اِطْعَامُ عَشَرَةِ مَسٰكِيْنَ مِنْ اَوْسَطِ مَا تُطْعِمُوْنَ اَهْلِيْكُمْ اَوْ كِسْوَتُهُمْ اَوْ تَحْرِيْرُ رَقَبَةٍ ۗفَمَنْ لَّمْ يَجِدْ فَصِيَامُ ثَلٰثَةِ اَيَّامٍ ۗذٰلِكَ كَفَّارَةُ اَيْمَانِكُمْ اِذَا حَلَفْتُمْ ۗوَاحْفَظُوْٓا اَيْمَانَكُمْ ۗ كَذٰلِكَ يُبَيِّنُ اللّٰهُ لَكُمْ اٰيٰتِهٖ لَعَلَّكُمْ تَشْكُرُوْنَ ( المائدة: ٨٩ )
Laa yu'aakhizukumul laahu billaghwi feee aimaanikum wa laakiny ya'aakhizukum bimaa 'aqqattumul aimaana fakaf faaratuhooo it'aamu 'asharati masaakeena min awsati maa tut'imoona ahleekum aw kiswatuhum aw tahreeru raqabatin famallam yajid fa Siyaamu salaasati aiyaam; zaalika kaffaaratu aimaanikum izaa halaftum; wahfazooo aimaanakum; kazaalika yubaiyinul laahu lakum Aayaatihee la'allakum tashkuroon (al-Māʾidah 5:89)
Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] – then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses [i.e., revealed law] that you may be grateful. (Al-Ma'idah  : 89)
1 Mufti Taqi Usmani
Allah does not hold you accountable for your laghw (ineffectual) oaths, but He does hold you accountable for the oath with which you have bound yourself. Its expiation is to feed ten poor persons at an average of what you feed your family with, or to clothe them, or to free a slave. However, if someone cannot afford, he has to fast for three days. That is expiation for the oaths that you have sworn. Take care of your oaths. That is how Allah makes His signs clear to you, so that you may be grateful.