(6) باب النَّهْىِ عَنِ الإِصْرَارِ، عَلَى الْيَمِينِ فِيمَا يَتَأَذَّى بِهِ أَهْلُ الْحَالِفِ مِمَّا لَيْسَ بِحَرَامٍ
Hammam b. Munabbih reported:
This is what Abu Huraira reported to us from Allah's Messenger (ﷺ), and he narrated a hadith and (one) of them is that Allah's Messenger (ﷺ) said: I swear by Allah, it is more sinful in Allah's sight for one of you to persist in an oath regarding his family than payment of its expiation which Allah has imposed upon him (for breaking the oath).
Reference: Sahih Muslim 1655
In-book reference: Book 27, Hadith 39
USC-MSA web (English) reference(deprecated numbering scheme): Book 15, Hadith 4071