Muwatta Malik #1063

Yahya related to me from Malik that Yahya ibn Said heard al-Qasim ibn Muhammad say, "A woman came to Abdullah ibn Abbas and said, 'I have vowed to sacrifice my son.' Ibn Abbas said, 'Do not sacrifice your son. Do kaffara for your oath.' An old man with Ibn Abbas said, 'What kaffara is there for this?' Ibn Abbas said, 'Allah the Exalted said, "Those of you who say, regarding their wives.'Be as my mother's back' (Sura58 ayat 2) and then He went on to oblige the kaffara for it as you have seen.' "