Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "By Allah! For one of you to persist in his oath concerning his family is more sinful for him in the sight of Allah than to give its expiation which Allah has made obligatory."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19852