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