Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever stubbornly persists in an oath concerning his family is a great sinner; even the expiation does not suffice for it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19853