Mishkat al-Masabih 15 Oaths and Vows كتاب الإيمان والنذور 24

وَعَن عقبَة بن عَامر عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «كَفَّارَةُ النَّذْرِ كَفَّارَةُ الْيَمِينِ» . رَوَاهُ مُسْلِمٌ

‘Uqba b. ‘Amir reported God’s Messenger as saying, “The atonement for a vow is the same as for an oath.” Muslim transmitted it.