Bukhari

Hadith Commentary:
A type of oath that comes to the tongue without any intention.
This is the view of Imam Shafi'i and the Ahl al-Hadith.
Imam Abu Hanifah said: If a person has a predominant assumption about something and then swears an oath upon it, then this is a vain (laghw) oath.
Some have said that a vain oath is one that is taken in anger or out of forgetfulness.
Others have said that what is meant is an oath taken regarding the abandonment of eating, drinking, clothing, and the like.

Hadith Commentary:
➊
This hadith shows that an oath which slips from a person’s tongue out of habit, without intention or resolve, is considered idle (laghw). This is because sometimes, in childhood, a person develops the habit of swearing oaths on every little thing, which becomes somewhat difficult to control.
Accordingly, it is narrated from Ibrahim al-Nakha’i (rahimahullah) that in our childhood, we used to be beaten for swearing oaths.

➋
If a person deliberately swears an oath that he did not do something, even though he actually did it, then this is called “false oath” (yamin ghamus), and it is counted among the major sins.