Musannaf Ibn Abi Shayba 7:372

12516. Abd al-Rahim narrated to us, from Yahya ibn Sa'id, from al-Qasim, who said: I was with Ibn Abbas when a woman came to him and said: "I have vowed to slaughter my son." Ibn Abbas said: "Do not slaughter your son, and offer an expiation for your oath." A man who was with Ibn Abbas said: "There is no fulfillment of a vow in an act of disobedience." Ibn Abbas said: "Has Allah not said regarding *zihar* (the pronouncement of divorce by likening one's wife to one's mother): 'And indeed, they say an evil word and a falsehood'? He prescribed for it the expiation that you have heard."