Musannaf Ibn Abi Shayba 7:464

Zayd ibn al-Hubab narrated to us, from Ibn Lahi'ah, from Khalid ibn Abi 'Imran, who said: I asked al-Qasim and Salim about a man who performed *zihar* (an oath of divorce by likening his wife to his mother) upon his slave-woman, and he did not find [the means] to free [a slave]. Should he free her? They both said: Yes.