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