Sunan Ibn Majah, Book 11, Hadith 2192 (In-Book) ⓘ

It was narrated that Hammam heard Abu Hurairah saying that 'Abul-Qasim said: "If anyone of you insists on fulfilling what he swore to (after learning that it is wrong) then it is more sinful before Allah than (breaking the oath for which) the expiation that has been enjoined upon him."

حَدَّثَنَا سُفْيَانُ بْنُ وَكِيعٍ، حَدَّثَنَا مُحَمَّدُ بْنُ حُمَيْدٍ الْمَعْمَرِيُّ، عَنْ مَعْمَرٍ، عَنْ هَمَّامٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، يَقُولُ قَالَ أَبُو الْقَاسِمِ ـ صلى الله عليه وسلم ـ ‏ "‏ إِذَا اسْتَلَجَّ أَحَدُكُمْ فِي الْيَمِينِ فَإِنَّهُ آثَمُ لَهُ عِنْدَ اللَّهِ مِنَ الْكَفَّارَةِ الَّتِي أُمِرَ بِهَا ‏" ‏ ‏.‏