Those who say that the expiation is to not fulfill it
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، عَنْ حَارِثَةَ بْنِ أَبِي الرِّجَالِ، عَنْ عَمْرَةَ، عَنْ عَائِشَةَ، قَالَتْ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " مَنْ حَلَفَ فِي قَطِيعَةِ رَحِمٍ أَوْ فِيمَا لاَ يَصْلُحُ فَبِرُّهُ أَنْ لاَ يَتِمَّ عَلَى ذَلِكَ " .
It was narrated from 'Aishah that the Messenger of Allah (ﷺ) said: "Whoever takes an oath to cut off the ties of kinship, or to do something that is not right, the fulfillment of his vow is not to do that."