The Prophet ﷺ said, "The major sins are associating partners with Allah, disobedience to parents, taking a life, and the false oath sworn deliberately."
عن النبي ﷺ قال: الكبائر: الإشراك بالله، وعقوق الوالدين، وقتل النفس، واليمين الغموس.
Bukhari narrated it in the Book of Oaths and Vows (6675) from Muhammad bin Muqatil, who said, Nadr informed us, Shubah informed us, Firas narrated to us saying, I heard Shaybi, from Abdullah bin Amr, and he mentioned it.As for his saying "the ghamus oath," it is said to have been named so because it plunges its perpetrator into sin, and then into the Fire. It is also said that the origin of this is that when people wanted to make a pact, they would bring a bowl and put in it perfume, or blood, or ashes, then they would swear while placing their hands in it, so that they might fully achieve what they intended by way of confirming their pledge. So that oath, if its holder betrayed it, was called ghamus because he had gone to the extreme in breaking the pact.Fath al-Bari (11/556).