Allah will not punish you for what is unintentional in your oaths,

Allah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation feed ten Masakin (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn . And protect your oaths (i.e. do not swear much) . Thus Allah make clear to you His Ayat (proofs, evidence, verses, lessons, signs, revelations, etc.) that you may be grateful.
{AL Quran, Surah Al Maeda (5), Verse 89}

Narrated Abdullah Radiallahu anhu : The Prophet Sallallahu Alaihi Wasallam said, "Whoever has to take an oath should swear by Allah or remain silent."
Sahih Bukhari, Vol 3, Book 48, No. 844:

Narrated Ibn 'Umar Radi Allahu Anhuma : that he found 'Umar bin Al-Khattab Radi Allahu Anhu in a group of people and he was swearing by his father. So Allah's Apostle Sallallahu Alaihi wasallam called them, saying, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."
{Sahih Bukhari, Vol 8, Book 73, No. 129}

Narrated Thabit bin Ad-Dahhak Radi Allahu Anhu : The Prophet Sallallahu Alaihi wasallam said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."
{Sahih Bukhari, Book 73, No. 126}

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