The Messenger of Allah ﷺ said, "Among the greatest of major sins are associating partners with Allah, disobedience to parents, and the false oath. No one swears a binding oath by Allah, including in it even something as small as a mosquito's wing of falsehood, without a black spot being placed in his heart until the Day of Resurrection."
قال رسول الله ﷺ: إن من أكبر الكبائر: الشرك بالله، وعقوق الوالدين، واليمين الغموس، وماحلف حالف بالله يمينا صبرا، فأدخل فيها مثل جناح بعوضة إلا جعلت نكتة في قلبه إلى يوم القيامة.
Tirmidhi (3020), Ahmad (16043), and Tahawi in Sharh Mushkil (893) all narrated it through the route of Yunus bin Muhammad, who said, Layth bin Saad told us, from Hisham bin Saad, from Muhammad bin Zayd bin Muhajir bin Qunfudh Taymi, from Abu Umama Ansari, from Abdullah bin Unays Juhani, who said, then he mentioned it.Tirmidhi said, "Abu Umama Ansari is Ibn Thalaba, and we do not know his name. He narrated hadiths from the Prophet ﷺ, and this hadith is hasan gharib."I say, Hisham bin Saad is the Madani about whom the scholars have made critical remarks, and his narration is hasan when he does not narrate alone, and this is one of those cases. He is corroborated by Abdul-Rahman bin Ishaq Madani, from Muhammad bin Zayd, from Abdullah bin Abu Umama, from Abdullah bin Unays, who said, then he mentioned it. Ibn Hibban narrated it (5563) through the route of Khalid bin Abdullah, from Abdul-Rahman bin Ishaq, with the same content.Through these two chains together, the hadith rises to the level of hasan. Ibn Hajar also graded it hasan in al-Fath (10/411).As for Abu Umama Ansari, it has been said his name was Iyas, and it has been said Abdullah bin Thalaba, and it has been said Thalaba bin Abdullah bin Sahl, and other names besides these have also been suggested. This is why Tirmidhi said, "We do not know his name," meaning the correct name.Tabarani narrated it in al-Kabir (1/250), in the biographical entry for Iyas bin Thalaba Abu Umama Balawi, saying, Muhammad bin Abdullah Hadrami told us, Ahmad bin Asim bin Anbasah Abadani told us, Abdullah bin Humran told us, from Abdul-Hameed bin Jafar, from Abdullah bin Thalaba, from Abdul-Rahman bin Kaab bin Malik, that he said, I heard your father say, I heard the Messenger of Allah say, "Whoever unlawfully seizes the property of a Muslim man by a false oath, that will be a black mark on his heart that nothing will remove until the Day of Resurrection."Ahmad bin Anbasah is Ahmad bin Asim bin Anbasah Abu Salih. Abdullah bin Thalaba is Abdullah bin Abu Umama bin Thalaba Ansari. Both of them are graded hasan in hadith transmission. Perhaps some of the narrators dropped Abdullah bin Unays from the chain, otherwise the hadith belongs to him.