Al-Ma'idah 5:89

La yu-akhithukumu Allahubillaghwi fee aymanikum walakin yu-akhithukumbima AAaqqadtumu al-aymana fakaffaratuhu itAAamuAAasharati masakeena min awsati ma tutAAimoonaahleekum aw kiswatuhum aw tahreeru raqabatin faman lamyajid fasiyamu thalathati ayyamin thalikakaffaratu aymanikum itha halaftum wahfathooaymanakum kathalika yubayyinu Allahu lakum ayatihilaAAallakum tashkuroon

Allāh 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 Allāh make clear to you His verses [i.e., revealed law] that you may be grateful.

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.