Musannaf Ibn Abi Shayba 7:467

Al-Hasan ibn Musa narrated to us, from Shayban, from Layth, from Tawus, regarding a man who performs *zihar* upon the mother of his child and does not find [the means] to perform expiation: He said: He shall emancipate her, and her emancipation shall be the expiation for his oath.