Leviticus 5:4
2 Or when there is a person who touches anything ceremonially unclean, whether the carcass of an unclean wild animal, or the carcass of an unclean domesticated animal, or the carcass of an unclean creeping thing, even if he did not realize it, he has become unclean and is guilty; 3 or when he touches human uncleanness with regard to anything by which he can become unclean, even if he did not realize it, but he has later come to know it and is guilty; 4 or when a person swears an oath, speaking thoughtlessly with his lips, whether to do evil or to do good, with regard to anything which the individual might speak thoughtlessly in an oath, even if he did not realize it, but he has later come to know it and is guilty with regard to one of these oaths— 5 when an individual becomes guilty with regard to one of these things he must confess how he has sinned, 6 and he must bring his penalty for guilt to the Lord for his sin that he has committed—a female from the flock, whether a female sheep or a female goat, for a sin offering. So the priest will make atonement on his behalf for his sin.
1 Samuel 14:24
22 When all the Israelites who had hidden themselves in the hill country of Ephraim heard that the Philistines had fled, they too pursued them in battle. 23 So the Lord delivered Israel that day, and the battle shifted over to Beth Aven. 24 Now the men of Israel were hard pressed that day, for Saul had made the army agree to this oath: “Cursed be the man who eats food before evening. I will get my vengeance on my enemies!” So no one in the army ate anything. 25 Now the whole army entered the forest, and there was honey on the ground. 26 When the army entered the forest, they saw the honey flowing, but no one ate any of it, for the army was afraid of the oath.
Notes and References
"... The very wording of Leviticus 5:1, “and he does not declare,” indicates that the person has committed wrongdoing by failing to perform a required action. This is not an act that violates an express prohibition, as do all the acts described in chapter 4. Rather, this sin lies in the non-performance of a positive command; hence, it is a sin of omission. The same is true for the sins described in Leviticus 5:2-4. This is clearest with the situation in verse 4. There, the scenario established by the text runs as follows: “...when he rashly swears an oath out loud for good or for evil, with respect to anything that one might utter rashly in an oath, and it is hidden from him, though he knew, and he becomes guilty....” In the ancient Near East, there were two basic types of oaths: the assertory oath and the promissory oath. The former usually took place in the context of a judicial dispute and served the purpose of asserting that a particular version of past events was the true version. The promissory oath, conversely, had nothing to do with the past. It was a promise to perform some action in the future. This is the type of oath envisioned by Leviticus 5:4. Interestingly enough, the act of uttering a rash promissory oath is not considered a sin and carries no penalty. That is, there is no biblical text, with the possible exception of Leviticus 5:4 itself, that implies wrongdoing on the part of any one who makes such an oath. To be sure, passages such as Ecclesiastes 5:2-7 advise one to be careful when making oaths, but Numbers 30:2, Deuteronomy 23:22-24, and Ecclesiastes 5:4 would all seem to indicate that once a rash promissory oath has been made, the oath-taker must fulfill it. (See also Judges 11:30-40) Thus, the sin that Leviticus 5:4 has in view may actually be something other than the utterance of a rash oath ..."
Wells, Bruce The Law of Testimony in the Pentateuchal Codes (pp. 69-70) Harrassowitz Verlag, 2004