Q.This type of swear/ oath where an individual swears by Allaah azza wa ja’al that this property belongs to him and he is lying (called yameenul gammooz in Arabic) the individual is asking if this is a major sin?
A. Yes it is a major sin, and the reason it is called gamooz is because it is as if the individual is drowning in the fire. It is that an individual would swear by Allaah azza wa ja’al claiming that this wealth belongs to him when in fact he has clear knowledge that it does not belong to him. It is a major sin.
Shaykh Saalih ibn Muhammad al-Luhaydaan (May Allaah preserve him) (Kitaab-ut-Tawheed Class 01.12.12) Riyadh.