Yahya related to me from Malik that Yahya ibn Said heard al-Qasimibn Muhammad say, "A woman came to Abdullah ibn Abbas and said, 'Ihave vowed to sacrifice my son.' Ibn Abbas said, 'Do not sacrificeyour son. Do kaffara for your oath.' An old man with Ibn Abbas said,'What kaffara is there for this?' Ibn Abbas said, 'Allah the Exaltedsaid, "Those of you who say, regarding their wives.'Be as my mother'sback' (Sura58 ayat 2) and then He went on to oblige the kaffara for itas you have seen.' "
USC-MSA web (English) reference: Book 22, Hadith 7
Arabic reference: Book 22, Hadith 1019