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

7- مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ عَنْ فَضَالَةَ بْنِ أَيُّوبَ عَنِ الْقَاسِمِ بْنِ بُرَيْدٍ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ سَأَلْتُ أَبَا جَعْفَرٍ (عَلَيْهِ السَّلام) عَنِ الأيْمَانِ وَالنُّذُورِ وَ الْيَمِينِ الَّتِي هِيَ للهِ طَاعَةٌ فَقَالَ مَا جَعَلَ للهِ فِي طَاعَةٍ فَلْيَقْضِهِ فَإِنْ جَعَلَ للهِ شَيْئاً مِنْ ذَلِكَ ثُمَّ لَمْ يَفْعَلْهُ فَلْيُكَفِّرْ يَمِينَهُ وَأَمَّا مَا كَانَتْ يَمِينٌ فِي مَعْصِيَةٍ فَلَيْسَ بِشَيْ‏ءٍ.

7. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from al-Husayn ibn Sa‘id from Fadalah ibn Ayyub from al-Qasim ibn Yazid from Muhammad ibn Muslim who has narrated the following: “I once asked abu Ja‘far, ‘Alayhi al-Salam, about oath and Nadhr (a commitment) and swearing about that which is an act of obedience to Allah. He (the Imam) said, ‘What is an act of obedience you must do it. If one makes a commitment then does not fulfill, one must pay expiation; but if it is an oath in sin, then it does not have any valid effect.’”