Al-Kāfi - Volume 7 > The Oath that Obligates one to Pay Expiation
Hadith #10

10- أَحْمَدُ بْنُ مُحَمَّدِ بْنِ أَبِي نَصْرٍ عَنْ ثَعْلَبَةَ وَحَدَّثَنَا عَمَّنْ ذَكَرَهُ عَنْ مَيْسَرَةَ قَالَ قَالَ أَبُو عَبْدِ اللهِ (عَلَيْهِ السَّلام) الْيَمِينُ الَّتِي تَجِبُ فِيهَا الْكَفَّارَةُ مَا كَانَ عَلَيْكَ أَنْ تَفْعَلَهُ فَحَلَفْتَ أَنْ لا تَفْعَلَهُ فَفَعَلْتَهُ فَلَيْسَ عَلَيْكَ شَيْ‏ءٌ لأنَّ فِعْلَكَ طَاعَةٌ للهِ عَزَّ وَجَلَّ وَمَا كَانَ عَلَيْكَ أَنْ لا تَفْعَلَهُ فَحَلَفْتَ أَنْ لا تَفْعَلَهُ فَفَعَلْتَهُ فَعَلَيْكَ الْكَفَّارَةُ.

10. Ahmad ibn Muhammad from ibn abu Nasr has narrated from Thadabah and narrated to us from those who narrated to him from Muyassir who has narrated the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘The oath that makes expiation obligatory is that which is about an act that is necessary for you to do. However, you take an oath not to do it but you do it. Expiation is not due on you because your act is in obedience to Allah, most Majestic, most Glorious. If it is something that you must not do and you take an oath not to do but you do it, then expiation is obligatory on you.’”