Musnad Ahmad 265:2

Yunus ibn Muhammad narrated to us, he said: Layth narrated to us, from Hisham ibn Sa’d, from Muhammad ibn Zayd ibn al-Muhajir ibn Qunfudh al-Taymi, from Abu Umamah al-Ansari, from Abdullah ibn Unays al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, among the greatest of the major sins are: associating partners with Allah, disobedience to parents, and the false oath (al-yamin al-ghamus). And no swearer swears an oath by Allah, while being compelled (to swear), and inserts into it (something false) the size of a mosquito's wing, except that Allah will place it as a black spot in his heart until the Day of Resurrection."