Al-Kāfi - Volume 7 > The Oath that Obligates one to Pay Expiation
Hadith #9

9- أَحْمَدُ بْنُ مُحَمَّدِ بْنِ أَبِي نَصْرٍ عَنْ جَمِيلٍ عَنْ زُرَارَةَ عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ السَّلام) قَالَ سَأَلْتُهُ عَمَّا يُكَفَّرُ مِنَ الأيْمَانِ فَقَالَ مَا كَانَ عَلَيْكَ أَنْ تَفْعَلَهُ فَحَلَفْتَ أَنْ لا تَفْعَلَهُ ثُمَّ فَعَلْتَهُ فَلَيْسَ عَلَيْكَ شَيْ‏ءٌ وَمَا لَمْ يَكُنْ عَلَيْكَ وَاجِباً أَنْ تَفْعَلَهُ فَحَلَفْتَ أَنْ لا تَفْعَلَهُ ثُمَّ فَعَلْتَهُ فَعَلَيْكَ الْكَفَّارَةُ.

9. Ahmad ibn Muhammad from ibn abu Nasr has narrated from Jamil from Zurarah who has narrated the following: “I once asked abu Ja‘far, ‘Alayhi al-Salam, about the kinds of oath for which Kaffarah (expiation) is obligatory. He (the Imam) said, ‘If something is obligatory for you to do and you take an oath not to do it, then you do it, you do not owe anything. If what is not obligatory on you to do and you take an oath not to do it but you do it, then Kaffarah (expiation) becomes obligatory on you.’”