Al-Kāfi - Volume 7 > The Intention for an Oath
Hadith #2

2- مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ إِسْمَاعِيلَ بْنِ سَعْدٍ الأشْعَرِيِّ عَنْ أَبِي الْحَسَنِ الرِّضَا (عَلَيْهِ السَّلام) قَالَ سَأَلْتُهُ عَنْ رَجُلٍ حَلَفَ وَضَمِيرُهُ عَلَى غَيْرِ مَا حَلَفَ قَالَ الْيَمِينُ عَلَى الضَّمِيرِ.

2. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from ’Isma‘il ibn Sa‘d al-Ash‘ariy who has narrated the following: “I once asked al-Rida’, ‘Alayhi al-Salam, about the case of a man who swears but his mind is on something other than what he has taken oath for. He (the Imam) said, ‘The oath is for that which is in one’s mind.’”