It is narrated from Sayyiduna Abdullah bin Unays al-Juhani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Among the greatest of the major sins are associating partners with Allah, disobedience to parents and the false oath (al-yamin al-ghamus). And whoever swears by Allah and deliberately mixes into it a lie even the size of a mosquito's wing, it becomes a stain on his heart until the Day of Resurrection."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13946
It is narrated from Sayyiduna Abdullah bin Unays (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The greatest of the major sins are associating partners with Allah, disobedience to parents and the false oath. By the One in whose hand is my soul, whoever swears an oath, even over a matter as trivial as a mosquito's wing, it becomes a stain on his heart until the Day of Resurrection."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13947