Surah 5

Surah 5:89

"God will not take you to task for a slip in your oaths, but He will take you to task for what you have pledged by oath. Atonement for it is the feeding of ten poor persons with the average (amount of food) which you feed your households, or clothing them, or the setting free of a slave. Whoever does not find (the means to do that), (the penalty is) a fast for three days. That is the atonement for your oaths when you have sworn (them, and broken them). But guard your oaths! In this way God makes clear to you His signs, so that you may be thankful."

5.89 – Atonement for it is the feeding of ten poor persons
The sense of atonement or “expiation” (kaffāra) in this verse seems to be making up for unintentional failings to keep oaths.

- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam