Musnad Ahmad 30:625

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Hammam, who said: I heard Abu Hurayrah saying: Abu al-Qasim (may Allah bless him and grant him peace) said: "If one of you persists in an oath regarding his family, it is more sinful for him before Allah than the expiation which He has commanded."